High school is like college in Japan. Its where you have friends and relationships before the real world and responsibilities. They also make it harder to get into college than through college, so once you finish high school your life is kinda decided in yhe way getting your college diploma is in the states. Baxk when they had jobs that didn’t need a college diploma a lot of people got married right out of high school. Plus it’s like the main time people can have money to spend but not enough to solve most of their problems, not be working all the time, and be stupid and borderline unable to function without necessarily being disabled. The just have growing up to do.

Tldr; high school is the cultural equivalent of college in Japan.

EDIT: also, most anime and manga is written for teenagers. Western adults are just not the target audience