Happens. In MGE's case it seems to me this particular super niche was carved out from the start. The thing is basically a cookbook for OCs based on a topic that was already somewhat niche. It's for the best, it means far less idiots trying to show up and fuck everything up and fewer still who show up without getting kicked out right away, but it also leads to a problem of actually finding people to get into it.
It's not like finding writers with a Touhou interest is all too easy either, at least of those who speak english. https://www.thp.moe/ exists, though I haven't touched the place myself it seems to be relatively frequently used for writing. Not sure anyone there is actually open to taking prompts or not though beyond the occasional site-wide writing events that are seemingly thrown around.
