I have no insider knowledge here, but Google tried to go the high route of working with carriers for years before giving up on their intransigence.
I suspect that Google's RCS is proprietary as a blunt instrument to prevent carriers from trying to either (a) undermine e2ee in some weasely way or (b) have the ability to pick and choose the pieces of the implementation they want to support. You either get the whole thing, with e2ee that you don't control, or nothing.
Sadly the lesson from Google, Apple, and Whatsapp here appears to be "cooperating with telecom carriers is a fool's errand".
I suspect that Google's RCS is proprietary as a blunt instrument to prevent carriers from trying to either (a) undermine e2ee in some weasely way or (b) have the ability to pick and choose the pieces of the implementation they want to support. You either get the whole thing, with e2ee that you don't control, or nothing.
Sadly the lesson from Google, Apple, and Whatsapp here appears to be "cooperating with telecom carriers is a fool's errand".