This is probably a minor aspect compared to its humongous fandom, but the games have an unreal amount of replay value.
Each game is like 40 mins long in total but takes an eternity to master. Now, why would you want to master such a game? The music and beautiful danmaku are probably more than enough reason to. Even if you lose the motivation to play for while, you could consume all sorts of touhou doujin media in the meantime: music, art, games, etc.
By consuming this media you simultaneously build up the motivation to play again, continuing the cycle.
I've been stuck in this lo