Kid here (34). My understanding is our generation grew up spending much more time looking at a screen in general. Gaming, socializing, following news, doing work, doing homework, talking to parents, talking to partner(s), reading books, painting, sculpting, paying taxes, managing finances, applying jobs, applying for visas... The list is too long to fit here. Participating in society demands more screen time than ever.
Dark mode makes screens blend with the environment better by emitting light where only needed. Since we can't reduce our screen time without any compromise, we try to optimize with the dials we are left with, in this case, pixel brightness.