Because when you get old the dark mode contrasts can give you headaches. I guess having to switch to light mode is is the worst thing about aging. People didn't know any worse before the internet, because they died in their 30's.
Aesthetically I prefer dark mode, but my eyes suck, so it's hard to read for long periods during the day.
Dark mode also gives that effect where if you look at it for too long, the light gets like imprinted in your fucking eye and when you look away you still see the imprint of the text like a CRT burn in on your retinas. I spend all day looking at code, and that shit would drive me nuts real quick.
My eyes are probably just fucked though, to be fair.