My son is a serious amateur weightlifter who’s been doing at it for years. His upper torso is fit, but certainly not “ripped.” It’s his calves that are freakishly strong. OTOH, my grandfather was a dairy farmer/construction worker. He had spindly bowed legs, but even in his 80’s, he still had very good upper body strength.
I suspect that humans develop only the muscles that they actually need.
One problem with having a lot of muscles is that the muscles add weight. That means it’s harder to walk, run, climb, etc. Imagine walking around with 20 or 40 extra pounds in a backpack. A heavy person would likely have some major disadvantages to hunting and nomadic travel.
One way that ancient humans would take down prey was to literally run it to death. Many animals can only sprint in short bursts and then need to rest. Humans would pick an animal from the herd and then just keep chasing after it until it fell over from heat exhaustion. So for something like that, it’s more advantageous to have a lean runner’s body rather than beefy muscles.
But we do still see swift muscle development in humans up until their 20’s. A teen boy will have no problem developing muscles with even a small amount of effort. That nearly-effortless muscle development declines as you get older. But if the important thing is to get you to an age when you can reproduce, that’s all you need.
A friend of mine is 6’9". He never played basketball (beyond gym class) because the only quality he had that would lead to success in the game was his height – he’s not athletic at all, and he’s a klutz. He also got extremely tired of people asking him if he played basketball. 
One of the reasons that humans haven’t evolved in any particular direction is that they don’t have to. Cultural evolution is the basket we have put our eggs into. We don’t evolve big canines, we invent knives. People in northern climates don’t develop fur, they develop parkas.
Cultural evolution is faster - advantageous changes can be adopted in a single generation or less. The Wright brothers make the first powered flight - seventy years later we land on the Moon. The wheeled plow and the horse collar lead to a population boom in Europe, and not because anyone’s genetics changed.
Plus, cultural evolution is directed. People see problems and they come up with solutions, without waiting for solutions to happen by chance. You want to move a heavy rock? Either wait fifty generations to become more muscly, or invent a lever. You want to throw a spear further? Evolve longer arms, or invent a spear thrower.
Humans have succeeded because we can talk, can think symbolically, and because we are good with our hands, with their long thumbs and the pincer grip of the thumb and forefingers, and the power grip of the bottom three fingers. And we’re bipedal, so we can routinely carry things with us when we go out hunting or gathering. Like spears and slings and digging sticks and hoes - and cell phones.
Regards,
Shodan