High end derailers shift easier, chains have lower friction (especially in high/low gears), spokes and rims are more aerodynamic and stiffer, ratchets have smaller engagement angles, brakes require less force and don't overheat, tyres have lower rolling resistance and better grip, seat posts have compliance to remove chatter etc., And this is before you get into eg. MTB specific features like chain clutches and suspension tuning, or for triathlon aero bars, for city bikes pannier clips and built in lights, etc
Nothing. Bike manufacturers and smaller builders are not trying to sell these bikes to you or to people like you. How hard is it to enjoy your "normal bike" and move on?
Some people ride their bicycles for thousands to tens of thousands of miles per year. They're going to buy what they want so that when they do ride, they enjoy it.
I feel like it’s just like people buying expensive sports cars, purely because they can, not because they actually need any of it.