RCS is all about controlling the messaging market — just like every other messaging network out there. Every implementation is proprietary and locked behind the implementer’s servers.
If you want something open source you want Signal. There are also other messaging solutions that you can self-host.
Big tech will never give you open access to their networks because that’s against the whole point.
SMS is the odd one out because it doesn’t require servers, and the reason it’s so open and universally supported is because back in the day the governments of France and Germany forced carriers to do it that way, and once it got popular it spread.