(Conjecture) It’s because of devs like me who stare at a screen all day and have tired eyes. It’s sized for ourselves. I keep my code at 16pt, and most websites at zoom of 150%. When I’m on my tiny laptop, I like to zoom to 200% (and usually need to go to full screen mode as a result). For context, my vision was recently tested and I’m slight farsighted. Still, just more comfortable that way. Please make your sites accessibility friendly, folks! Much appreciated.