Here's the problem, RCS isn't a truly open thing and Google kind of maintains a lot of the software that even carriers use for it. It essentially opens the door for the tech companies to take over yet another big chunk of the carrier services and tap into more user's data at the network level.
In June 2019, Google announced that it would begin to deploy RCS on an opt-in basis via the Messages app, with service compliant with the Universal Profile and hosted by Google rather than the user's carrier, if the carrier does not provide RCS
In October 2019, the four major U.S. carriers announced an agreement to form the Cross-Carrier Messaging Initiative (CCMI) to jointly implement RCS using a newly developed app. This service was to be compatible with the Universal Profile.[33] However, this carrier-made app never came to fruition. By 2021, both T-Mobile and AT&T signed deals with Google to adopt Google's Messages app.[34][35][36] In 2023, T-Mobile and AT&T agreed to use Google Jibe to implement RCS services, and in 2024 Verizon agreed to use Google Jibe.
Apple stated it will not support Google's end-to-end encryption extension over RCS, but would work with GSMA to create an RCS encryption standard.