I guess they reckon that because microgravity makes one weaker unless they’re constantly working out, macrogravity would make one stronger.
I mean, it would.
In most of the various written pieces of Trek lore, the hybridization requires advanced medical assistance / genetic engineering. Done more by other races than Terrans, due to social fallout from the Eugenics Wars. But yes, “The Chase” did indicate the shared heritage made it much easier, and in some cases possible even without medical intervention.
Don’t forget the Horta! FTR, a lot of less humanoid or more elaborate options exist/existed in the versions that didn’t have to be filmed. TAS, Lower Decks, and of course, the various novels and comics.
Geek points, but in the FASA Star Trek TT-RPG, humans are indeed very median, but have one exceptional stat, far superior to any other race: Luck. 
Prior to ST: Enterprise and that twist on the lore, the “Klingons” seen in that and similar episodes were sometimes handwaved as Klingon/Human Fusions - hybrids used with the expectation that they’d be able to gain an advantage in thinking and acting more like the enemy. While discussed in some of the novels, it wasn’t ever a super popular explanation, and the Enterprise variant on it makes at least slightly more sense.
Back to the OP in more detail. Yes, the Klingons are substantially stronger than humans, but while they are a warrior culture, they are almost always depicted (with few exceptions such as Worf) as fighting in either a ritualized or thuggish manner. Which means humans, who presumably don’t depend on their natural gifts but are better trained in ‘practical’ hand-to-hand manage to give as good as they get.
For that matter, Klingons, much like their arguable ‘real-world’ analog, the Russians, are often presumed to be intoxicated, even when on duty. Much less at a bar even if in human space.
Not all alien races are stronger than humans. But the Tales Of Trek are abridged. Stories of planets that are dull and plodding don’t make the cut.
There is a metabolic cost to every system in your body. One of the costliest is the human brain. Spending less energy and resources frees up more for supporting that human brain. Humans outsource a lot of stuff to our technology - we don’t have to burn as many calories to stay warm as comparably sized mammals because we have clothes and fire, we don’t have to “invest” more in our teeth and digestive systems because we have knives and fire to cook with, and we don’t have to spend as much on stronger muscles because we discovered things like levers and spear throwers. Our weaker muscles aren’t a “defect” , they’re a necessary trade-off for our metabolically expensive brains which are arguably our best asset given how they’ve allowed us to spread like a plague across the planet.
What, no love for the Hortas? Although I understand there was a guy underneath all that stuff creeping along to make the Horta move. Hortas were an excellent non-humanoid intelligent alien.
In higher gravity, a humanoid species would need stronger muscles just to lift its own weight, and faster reflexes because everything would fall faster. Perhaps in practice humanoids on a high-gee planet would be short and squat to reduce the amount of extra weight they would need to carry, and the distance their heart would need to pump their blood to their brain. Being smaller would also help when falling over in high-gee.
All things considered, a high-gravity humanoid is probably unlikely to evolve - I would expect macrogravity species to resemble small elephants rather than Tolkienesque dwarfs.
I don’t recall the Hortas being a species encountered on screen more than once (the episode “Devil in the Dark”). Whereas the Tholians appeared or were mentioned in multiple episodes, including the Enterprise series and TNG. (And of course, the Gorn appear repeatedly.)
One major trade-off for humans’ low strength is that the sort of musculature and skeletons we have make us a lot better at throwing things than the other apes. Once you’re good enough at tool-making to have effective thrown weapons, that’s a big asset.
In Hal Clement’s classic Mission of Gravity, at one point the human character asks one of the aliens (a native of a planet with many times the gravity of Earth) how long it takes an object to fall when dropped. The answer is “No time. Once it’s dropped, it’s on the surface.”.
They take a lot more care than humans do to never drop things in the first place.
Why would anyone expect other humanoid species to have the same strength???
A commonly believed myth.
First they are not that much stronger actually, and there is no such “genetic defect.”
https://www.pnas.org/doi/10.1073/pnas.1619071114
Most of the difference is the relative fraction of fast twitch fibers. That makes for more firing quickly all at once, higher peak strength/power, but less endurance and less fine motor control.
No cite readily available but I’ve previously read that motor action units reflecting innervation are also consistent with that: humans are evolved for more varied fine control, which needs more brain real estate, chimps to fire all the big muscle at once.
That is, FWIW, part of human strength gains, the neuromuscular adaptation to fire all at once for a very specific task.
A Vulcan may have a brain able to switch gears from very fine control to using all the muscle all at once, reflective of greater brain capacity.
The cite.
But the other more recent cite does dispute that hypothesis.
So, it’s not that chimps are able to tear off your arms and eat your face, it’s that they want to?
Showing we’re not that dissimilar, I can only thank my decades of social conditioning to prevent me from eating the face when I was accosted by Trump fans outside the local grocery store yesterday trying to grift money / signatures to support him during this unjustified attack. Back to Trek, a Romulan or Klingon with superior strength an no social expectation of civility (socially, very possibly the reverse!) could also destroy a human without training and with equal lack of care.
Seriously though being stronger is not a huge advantage to the space-faring, phaser/disintegrator wielding sophonts of that setting. Outside the fact that the various characters are often getting themselves into troubles that more rational beings would have avoided (yes, that’s the script making it happen) - it would rarely be an advantage.
Another factor is that they aren’t remotely humanoid, so can’t easily fall over or drop things very far.
You first should realize that Gene Roddenberry knew very little about science, no more than an average high school graduate of his time. (He graduated from high school in 1939, I believe.) He got most of the image he had of what aliens would be like from watching 1950’s and early 1960’s science fiction movies and from television shows like The Twilight Zone. Aliens in those things weren’t remotely plausible scientific guesses at what non-Earth life would be like. They were attempts to model them after the creatures of horror films like werewolves and vampires. The plots of many Star Trek shows were political allegories, as were the plots of The Twilight Zone (since Rod Serling had to try to make political points by twisting a political story that American television networks wouldn’t allow onto their networks by making it about aliens or horror creatures instead). Roddenberry had to make the alien races look too close to humans so that people would watch it. In recent years, Star Trek fans and writers have had to come up with various bizarre explanations why the aliens looked so much like humans. The aliens of the early Star Trek shows Roddenberry created had to have powers that humans didn’t because otherwise fighting them off would be no harder that killing a group of humans.
Slight Hijack: Thomas Magnum… Tall, handsome, athletic Navy Combat Veteran used to get his ass kicked pretty regularly…
The Cylons from the original Battlestar Galactica — they somehow drove the human race to near extinction, but you never saw them win a fight. Even in the episode where all the human pilots were sick, so untrained substitutes had to fly the vipers.
There were lots of encounters with weaker aliens, but those mostly didn’t make it into the historical documents. Mostly.
Hell I know of a person killed by a swan and I would not want to up against a raccoon. Pretty sure I’m stronger than either.
Pretty no one here imagines that Roddenberry and the early writers were doing hard science fiction based on true xenobiology informed speculations. Of course it is easier for viewers to identify with motivations of aliens that mostly seem like us, and easier to do the make up. Energy cloud and rock based aliens are best as one offs . But there is more fun to be had in making it work in-universe!
You could be weak but smart. Or strong but dumb, or have some other flaw that would defeat you. They had a couple of times (Organians) where the smart aliens were also stronger and/or morally superior, but that’s not a plot you can use all that often. Weak and dumb is just boring or “there must be a catch.”
Relevant, yes. But it seems people disagree on exactly what that stuff did.
Human characters drank this green stuff that allowed them to take on Klingons hand-to-hand. But it’s unclear if it actually made them physically stronger, or was more like a huge adrenalin spike.
Most of the arguments though were about whether it was a good idea to introduce that. A lot of people agreed with what I said above about hand-to-hand fighting being boring, and would have preferred a more clever solution.