To be honest as a non-american, americans seem pretty obsessed with high school as well.

That's for the simple reason that this is their main target-group. Anime is for kids, because adults don't have time to waste for them. Adults read manga, on train or break-time. And many manga have mature characters and topics.

Of course are there are also manga aiming for kids and anime targeting adults, and in the last decades things have changed, because technology as also society changing. Also today there are strong tropes, sophisticates nerds, etc.

Besides what the sibling poster said about target audience—these days, a lot of it's down to the fact that many manga/anime are being adapted from commercially-published(!) novels written by high-schoolers.

Yes, Japanese book publishers are signing deals with teenagers these days. I think with the justification that they can better write things their peers want to read. I've got to believe they have sales numbers to back that up.

But high-schoolers are still uncreative fiction-writers, writing mostly high-schooler characters in high-school-equivalent settings. I guess the publishers don't complain as long as the books sell.

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Though, honestly, what's really going on here is that everyone is mostly uncreative when writing voluntarily under no particular artistic constraints. Tangential Sturgeon's Law rant about fiction-writing (from the perspective of an editor reading through a slush-pile) follows:

People mistake the advice of "write what you know"[1] for a recommendation to just "write your own life-story, gussied up a bit."

For every one amateur writer who constructs a novel fictional setting, there will be nine writers who will just pastiche together the last three books they read; and 90 more who will won't bother with a novel setting at all.

For every amateur writer who constructs an interesting character "from whole cloth", there will be nine that just re-skin characters from the stories they like, and 90 that just attempt to recapitulate the mostly-mundane people in their own lives.

For each amateur writer who starts with the intent to write a novel conflict/scenario, letting the work define its own genre; there are 999 writers who will instead choose a genre to work in, and then regurgitate exactly the same scenario tropes they've already seen from works in the genre. (Many authors seem to think that scenario tropes are what genre is.)

Professional authors aren't any better on their own, but usually they'll either have a publisher or editor who will push them to work outside their comfort zone; or they'll have written the same thing enough times that they'll finally get bored of it, and enter a more experimental phase.

[1] If you're curious, "write what you know" is supposed to mean "if you want to include X in a story, then go research+experience X. Embed yourself in the world of X, like a gonzo journalist. If you want to write a story focused on a real-world place, you've got to live there for a while; if you want to write a story focused on a certain industry, you've got to have actually worked in that industry for a few years." It's not supposed to hold you back from writing what you don't know; it's supposed to push you to go know it!