Because it's a carrier-owned upsell feature. It's nuts that people even consider a non-Internet protocol that applications can't implement, and users have to get permission to use — from the same carriers that charge for bytes of crappy size-limited MMS messages in the same ballpark as delivery of physical letters.
SMS is carrier owned and had an API. What you're saying doesn't surprise me, but I guess an RCS client written by Google and can't talk to iPhones isn't exactly an upsell.
Yeah, I don't know what the Google/Jibe/carrier relationships are, but Google made an exception for Samsung[1], which I suppose could've been carrier approved too. Looking more at it, it seems like Google Messages is still the RCS client, and it's exposing messages to Samsung's app.
Honestly, it's kind of hard to blame Signal and/or Google if carriers are involved here. I mostly use Signal and Element, so if those two had a single client, that'd be fine. But, I think Signal probably still favors a centralized system, and they have added ongoing call transfers from and to mobile/desktop, and now Stories and the money transfer (not a fan), so I suppose it's up to the users to choose.
