It is fucking compatible. Messages falls back to the most compatible option between platforms and carriers: SMS/MMS. RCS has been a shit show, even Google has had to run their own parallel infrastructure for it just to avoid the implementation problems of carriers. Google also has proprietary extensions for RCS for E2EE which only works on Google's infrastructure and only between clients running Google Messages.
You're trying to excoriate Apple for not supporting an extremely poorly implemented "standard" (RCS) or investing tons of money fighting with carriers and partners trying to create some new "standard". It's absurd.
If Apple tried to support the RCS clusterfuck that would be a third protocol supported in their Messages app. Even if it was some new standard hammered out with Google and carriers there would still be an SMS/MMS fallback for the literal billions of handsets that won't support whatever new standard.
It's the definition of a walled garden. Google's messaging services aren't limited to Pixel devices or even Android in general, whereas Apple has made theirs available only on devices they manufacture.