It used to be that a car’s plugs needed to be changed every 20-40k miles. Now, I don’t remember the last time that mine have needed to be changed. Is it better engine quality, better plugs, or some combination of the two? What’s different?
Most manufacturers are now selling their cars with so-called “extended life” spark plugs intended for 100K miles. These are iridium or platinum tipped instead of the old copper tipped plugs.
I would imagine that with everything being computer controlled, the addition of O2 sensors and replacing a carburetor with fuel injection all helps reduce the amount of unburned gas that can foul the plugs. Better tolerances and emissions probably helps keep oil out of the combustion chamber. And getting rid of a distributor probably helps make the the spark, as well as the timing of the spark, more reliable.
Also, once you get into the mid 90’s with OBDII showing up (and I’m sure with plenty of cars before it as well), if your engine was misfiring the CEL would show up and alert you to the problem.
On top of everything else, changing the plugs seems like it was just part of a ‘tune up’ and was easy routine maintenance that just about anyone can do. In a time when so many people did their own oil changes, doing the plugs once a year didn’t add much time or money to the project.
Nowadays, with cars getting more and more difficult to work on in your backyard, people tend to go to quick lube or dealerships to get their oil changed and they’re not likely to get the plugs changed unless there’s a good reason for it. My Honda doesn’t even recommend changing the air filter all that often anymore.
Or, the plugs are better and don’t wear out as fast as they used to.
When I was a kid my dad would sandblast the plugs clean at work. In my lifetime of driving I’ve never had to change or clean plugs.
It’s a good thing too. I used to do all my own maintenance.
My Wife has an '18 Subaru, and I a '19 Toyota. I’ve looked under the hood. Yep, that must be the engine.
I remember standard tune ups included new plugs, points & condenser. Adjusted the timing with a timing light.
Even had to gap the plugs with a feeler gauge.
A lot of service stations offered tune ups and oil changes.
yabob and Joey P have the right answers. Better plugs and better cars. 100,000 mile spark plug change intervals are normal.
The only car built after 1996 that I’ve had to change the plugs on was my friend’s Mazda Rx-8. Those things burn oil like two strokes and their plugs get fouled easily. Other modern cars just don’t have that problem.
Going back long enough what killed spark plugs was leaded gas. The lead would leave a conductive coating on the plugs that eventually shorted them out. So unleaded gas alone meant much longer plug life. After that all cars became fuel-injected with oxygen sensors which further extended plug life as the air-fuel mixture was more precise. And finally (as mentioned above) traditional copper plugs were replaced by platinum or iridium tipped plugs which wear slower.
But with all that said, personally I’d still replace plugs every 50-60K miles as they look pretty bad at that point. They still work, but the engine runs noticeably smoother after you change them out.
Shoot, I remember doing all that, but on motorcycles. They lag a bit behind the automotive state of the art. I think you can still buy a bike with carburetors. But almost certainly not points ignition.
How about lawn mowers? Will I be replacing the spark plug on my basic push mower any time soon?
Every 10,000 miles. 
I just replaced the plugs on my Prius. It is a 120,000 mile replacement interval. And a good thing, too, because they aren’t easy to get to. Requires first removing the windshield wiper motors! Took a couple hours.
But at 120,000 mile replacement interval, I guess they don’t really need to be convenient to get to.
They don’t accumulate nearly as much run time as cars tend to. Small engines (i.e. the simple, cheap pieces of crap you find on lawn mowers) have a design life that’s on the order of 100-200 hours, compared with a car engine that might take 2000 hours to hit 100,000 miles. And it takes many calendar years to hit 200 hours on a typical lawn mower. Mine is now 17 years old, and I’ve never changed the oil or the spark plug. Still runs like a champ. The wheels will wear out first.
It started with the switch to unleaded gas, resulting in conventional plugs that used to last 10,000-12,000 miles now lasting 30,000. The next big jump was platinum electrodes, bringing it to 60,000. Iridium has doubled it again to (often) 120,000.
This is something to remember. Your car might still be running okay on old plugs but it could run better with new ones. Smoother, better gas mileage, less emissions, etc.
The computer-timing tends to partially mask performance issues that slowly accumulate over time which most drivers won’t notice.