It seems like every other message gets downgraded to SMS.
Then it has to accomodate every other intersted party, many of which hate each other. Apple has always been a bit of an odd duck ("Think Different" has been internalized for some time), but Verizon actively hates OTT messaging as they can't charge for it. Samsung would rather run their own RCS implementation to create and advertise "Samsung RCS", and Google can't push too hard without getting EU attention for antitrust (again).
RCS has been stuck in limbo-hell for years for multiple reasons, none of which are easy.
This is a problem no other vendors have, and is solely caused by Apple.
https://www.androidauthority.com/android-iphone-rcs-messagin...
That's not Apple's fault per se, but of course, they contribute to it. They should open up the iMessage protocol.