the highschool is just a setting, but if you look at isekai anime you'll realise that what really matters is a talentless guy who surrounded by girls that adore him and who cant stop winning, big mean antagonists that get defeated and a charming friendgroup. it just happens that a highschool has all these things and is an infinite source of inspiration for creators, also beside an uneventful cubicle job its something all japanese poeple can relate to