Just saying "fanworks" is fucking stupid if you're not looking at why people made Touhou fanworks in the first place. Why wasn't the big 2000s doujin craze, like, Seihou, or any other doujin game of the period?

I think it a lot of it has to do with the way in which ZUN's stories basically run on the level of pseudofolklore, like Yuyuko stealing Spring from Gensokyo, or Suika punching the moon, told in this limited way where you only get glimpses of dialogue between characters which you repeat over and over and as you learn the game and each boss's pattern. Which itself is a sort of process of understanding the nature of their character through repetition. Point being, 1CC'ing any of the mainline Touhou games is like playing an arcade game recreation of being told a folktale over and over again as a child, which naturally leads to strong impressions of the boss characters while leaving the world and logic of Gensokyo vague and magical enough to offer itself to fan interpretation.