The tiny company behind Signal, and a bunch of even smaller companies making messengers, manage to make their app work just fine on iOS and android (and most of them on desktop too, some even in the browser).

For some reason, the (second) richest company on the planet, which has a messenger app, is incapable of making an android app for their messenger and chat protocol.

There's a technological issue, and it's nothing to do with RCS or SMS, it's that iMessage for some reason doesn't have an android version.