Because we've been stuck with the same bicycle UX for like 150 years now.
Sometimes shit just works right, just about straight out of the gate.
By the 1870s we'd pretty much standardised on the "Safety Bicycle", which had a couple of smallish wheels about two and a half feet in olden days measurements in diameter, with a chain drive from a set of pedals mounted low in the frame to the rear wheel.
By the end of the 1880s, you had companies mass-producing bikes that wouldn't look unreasonable today. All we've done since is make them out of lighter metal, improve the brakes from pull rods to cables to hydraulic discs brakes, and give them more gears (it wouldn't be until the early 1900s that the first hub gears became available, with - perhaps surprisingly - derailleurs only coming along 100 years ago).
In the last 20 years alone we've seen introduced or popularized:
Carbon frames, carbon wheels, disk/hydraulic brakes, expanded cassettes (2x11, 2x12), electric shifters, aerodynamic wheels, string spokes, and a boat load of different tires (for different levels of comfort, speed, durability, grip) for whatever you are doing. That's road bikes.
For mountain bikes you have all of that (minus aerodynamic stuff), dropper posts, 1x drive trains (1 chainring, 12 gears on the cassette, these are so good people want them on road/gravel bikes too) plus a slow evolution of geometry that completely changes how the bike feels on different terrain (also making them safer on dangerous terrain), plus a slow march from incredibly heavy builds to today's lightweight builds which are still more than capable of handling downhill. And in that same time-frame you'll see them going from 26" to 29" wheels, which results in a massive difference in the way the bike rides, and also the bike's ability to go over obstacles. And tubeless tires are now popularized on MTB, which means you can run lower pressures for better comfort and traction, and you also spend a lot less time futzing with holes in tubes.
Not to mention... E-bikes. There's just been so much going on. A lot of that might sound like it does a bunch of nothing, but all of it contributes heavily to how the bike feels and performs overall. Carbon frames and wheels for example aren't just about weight savings, they make the bike more rigid (which changes how it responds to inputs) and reduce vibration making the bike more comfortable overall. If you were somehow blindfolded and put on a bike from 20 years ago versus one from today you'd be able to tell. Doubly so for 40 years ago.