"The answer is this: They dont want to add RCS support or spend the time to do it."

... confirmed by Signal in their discussion thread:

"... and Signal can’t add RCS support because there’s no RCS API on Android. Honestly, the days of any third-party SMS app are numbered."

I guess I misunderstood RCS. I thought the whole point of RCS was to be used on Android and to allow disparate third parties to use it as an open standard.

Where is the RCS API if not on Android ? Who is supposed to use RCS ?

If they said that up top I think many more people would be accepting of it.

Google doesn't allow 3rd-party apps to access the RCS API.

There are no public RCS API's. No one (besides an OEM) can make an RCS app.

Google also restricts their specific flavor of RCS (or at least they did awhile ago). I wanted to keep using Textra SMS but they never let Textra into RCS land.