A lot of my friends from back home ask this question but it's always stacked to begin with because they've come in with a mindset and most of their conclusions already decided so my apologies because if I sound jaded it's because I am. I don't see it as being 'stuck in high school' and certainly not 'arrested development,' partially because the huge majority of Japanese people aren't into the kind of pop culture that we've come to associate with Japan. It's just that Japanese pop culture outside of anime and idols and shit like that, is pretty boring, so there's a filter that means in the west you only see the crazy stuff that shocks your parents and starts 294 threads a day on Gaf.
I mean it's not like I really know the answer anyway. I could take guesses as to why high schoolers are often the symbol of Japan, like the fact that high school life is seen as the highlight of your lifespan for many people, so it's always going to be seen as this ideal that you only achieve for three years of your life and after that it's gone. High schoolers are seen as having their whole lives ahead of them, and children in Japan get a lot of "support." Older people definitely feel the changes and don't approve of things that younger people do like dying their hair brown or wearing flashy nail art, but since there's a large respect for the elderly, indeed tons of my students say they want to go into senior care as a profession, instead of resentment going both ways, there's a fair bit of admiration. Not that it's perfect, mind you, but when I compare it with how the young and the old are always battling with each other in my country, it sticks out a lot to me.
It probably also relates to the workforce, in that a lot of people work menial jobs for long hours where they limit their interaction only to a small group of coworkers around them who they don't necessarily like. The happiness index is really low in Japan, despite the quality of life being high because of this, I think. People have one or two hobbies that give them joy and of course they raise families, but when 11 hours of your day is spent in the office, you look back on your high school years and it just seems much brighter.
The single biggest reason why high schoolers are the main characters in anime, video games, edgy movies, etc. is because the audience for that stuff is all young. So I could go into a lot of hypotheses, but ultimately it comes down to the audience, I think.