They tried to make a bazillion messenger apps, all of which failed, and now they try to piggyback on an existing standardized protocol but don't expose any APIs for other apps.
So today it's either Google or Samsung messages and Jibe, otherwise RCS is essentially useless. From what I know Facebook wanted to get on board at some point, I'm not sure why this didn't work out.
The fact Google publicly whining about the iMessage lock-in is pretty rich.