Some do, but what Google rolled out in Android Messages is their own implementation unrelated to the carriers. Ostensibly so it works regardless of carrier, but what they rolled out is a semi-proprietary implementation that only works on their app. Ergo if you use a third party texting app, no RCS. So it’s a sort of “Android iMsssage” thing anyway. Apple plans to implement Google’s version, again sidestepping the carriers.