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.
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.
[1] https://www.sammobile.com/news/google-messages-rcs-integrati...