It's the evolution of SMS/MMS; development started 18 years ago, in 2007. The modern spec is based on that with a whole bunch of additional revisions for things

It's the evolution of SMS/MMS; development started 18 years ago, in 2007. The modern spec is based on that with a whole bunch of additional revisions for things like video calling and transferring money. It was designed long before major messenger apps had e2ee in the first place. Had it been designed with the security practices at the time, the protocol would've been ossified to the point of being practically insecure by today's standards. In a sense, the fact nobody cared about it until the spec was old enough to drive is actually good for users. The GSMA which designs RCS also serves the needs of government agencies that are tracking (international) criminals, so I bet there must have been some pretty strong opposition against E2EE in the official spec. Frankly, I'm surprised they're even putting it in the spec.