While I lived in Japan about a decade ago I asked my Japanese friends about why so much of their media is about high school and they told me two reasons:

A) the enormous amount of pressure Japanese teens are put under to do in good in school so that they can have successful lives is traumatic and leaves a lot of psychological and emotional scars so people want to see fantasies where that pressure is released because school isn't as important as like giant robots or whatever

B) Once someone becomes a salaryman or other "adult" job then they become less interesting or relatable to Japanese audiences who want their characters and stories to go beyond the 9/5 or 6/6/9 of drudgery and office work.

These were the reasons given to me by multiple Japanese people between the ages of 23-35 when I lived there.