Is it because no one else has tried implementing the RCS standard or because Google has some proprietary hold over it?

No one else has tried implementing the RCS standard.

There just aren't any open-source Android libraries for RCS out there, much less anything in AOSP.

https://github.com/search?q=rcs+android&type=repositories

Apple has RCS, compatible with Android, at least in the EU. You don't get the blue bubble though.

I think they’re asking about on Android. The point is whether there’s an alternative RCS client to run on GOS.

Which itself is not guaranteed to work even on ”stock” Android, let alone GOS — which multiple people (myself included) have been experiencing firsthand.