Since a PC was a big box of parts anyone could manufacture one. A modern phone is much more complicated.
As to why there aren’t a plethora: the market doesn’t demand it that much. The people doing it aren’t wildly successful. Perhaps that’s changing (I hope so) but I know very few people outside this community who have ever thought “I wish I could have a third party version of Android”.
Edit: I am not saying just user replaceable. I mean standardized so the same cells in a 2024 phone also works on 2025...