The only thing proprietary in the early PC architecture was the BIOS. Everything else was pre-existing architecture from third parties, there was nothing to keep a lid on.

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”.

Even the batteries are not interchangeable on phones. You'd think all phones should have the same exact battery, that this kind of standardization is beneficial for phone manufacturers as it helps them bargain with their parts suppliers but no for whatever reason we can't have that.

Edit: I am not saying just user replaceable. I mean standardized so the same cells in a 2024 phone also works on 2025...