I don't know much about RCS other than that carriers need to be involved, but there's a way to not involve them, which Google did for a while using some kind of compat-service (jibe or something?), and then they stopped doing it.

Never seen RCS working on any carrier ever since. I don't understand why Google couldn't just continue doing what it was doing. Why require carriers?

The intended purpose of RCS was to be federated between carriers, just like SMS. If you centralise it then it's just a worse version of any of Google's abandoned chat apps. But there's a realpolitik reason to insist on centralised RCS: because it's already got the reputation of decentralisation, it makes it look like Google is better than it actually is.