Apple has no obligation for users outside of their ecosystem. Apple saw the landscape of carrier messaging being terrible, and they made iMessage to help their customers communicate with one another better, while continue to maintain support for basic carrier communication. They have now updated to offer RCS, the current modern carrier messaging standard, which as demonstrated is still fragmented and outright garbage.
There is a Google proprietary protocol that’s based off of RCS, but as demonstrated by the Android market, even Android devices doesn’t do that — so Apple isn’t likely to (and frankly shouldn’t) do it to give more information to Google (even on the alleged promise of E2EE, it allows Google to know who is communicating with who at what time, and potentially roughly where via cell tower origination).
Apple is not a charity and has no need to open up their proprietary protocol designed to better their clients’ communications to non-clients. Want to make a phone call? Pay your carrier. Want to have electricity? Pay your power provider. Want to use iMessage? “Buy your mom an iPhone”.