blind_dead_mcjones said:
because in recent years since haruhi became a big hit, the studios and producers discovered that high school romantic comedies and borderline creepy moe are what sell to the hugely repressed social outcasts with massive disposable incomes that are present in their society.
That was a thing before, it's just gone through a recent resurgence thanks to Haruhi. Sort of like how the "schoolboy wizard" genre has always been around, but Harry Potter infused it with new life and an explosion of popularity.
OT:
Most of the (probably more valid) points have already been made, but for me personally, the appeal of high schoolers having adventures is the fact that I effing HATED high school. Worst four years of my life, in a miserable environment surrounded by incredibly limited minds supported by a degrading system whose administrators are criminally ignorant where they're not deliberately harmful.
If I'd had the chance to go demon-slaying or reality-hacking or even just find true love, I'd have been on it like Yahtzee on tortured comparisons (see what I did there?*) I'm guessing that at least a few anime/game directors/mangaka have the same notion: it's wish fulfillment of what high school SHOULD have been like.
*No disrespect to Yahtzee intended.