For one, Google never wanted to implement or run RCS, the carriers were supposed to do that specifically to prevent the fragmentation issue. But they couldn't get off their ass, even after Google spent years pushing them to do so.
For two, even after Google said fuck the carriers I'll do it myself, Apple was invited to participate in its implementation and Apple refused. They could have worked with Google to implement RCS across all devices. They didn't because they want to keep people locked into their ecosystem. They had a great opportunity to ensure all the privacy and encryption features were implemented how they liked.