Google is slacking here and I hope Apple's involvement in RCS will help to move this forward. Samsung Messages also does not support Google's E2EE even though it supports RCS and pretty much all of the user-facing features Google Messages provides. Based on Google's whitepaper [1] about their E2EE support, I imagine it's because of the identity service they use for key exchange being centralized and internal (when really the identity service a contact uses should be an RCS capability in the extended contact system [RCS terminology here], and they should interoperate).

[1] https://www.gstatic.com/messages/papers/messages_e2ee.pdf