Charitably: They envy the carefree life, meaningful connections, and openness with emotions the characters exhibit. They think that kind of happiness has been denied to them by communists/the wokes/being born in the wrong generation/minorities.

Uncharitably: They're pedos and K-On has a more realistic depiction of adolescent girls than most Japan pedo slop

I watched the first season years ago and yeah, what machiabelly said. All I know for certain is if you see a K-On pfp you're about to read the most racist shit you've ever seen in your life.