Kaeru no Kuni brought me back here, but for me what coylie says captures it exactly. And nez nails it every time.
Though what Marisa said on pages 13-14 here works for the story, gensokyo can't, and doesn't, carry on all the traditions that go to the wayside in our world. It's too small a place to contain the world's, or even just Japan's, spiritual and ritual history. I'm not even sure if there's a canon explanation for how cultural practices enter gensokyo, if they even do. It may just be the culture is locked from the late 1800s and only forgotten objects, like obsolete cell phone towers (or arcade cabinets!), can enter (and people, gods, buildings, etc).
But, I haven't finished wild horned hermit or silent sinner in blue yet, if they explain more