RCS' rollout has been a complete shit show. Carriers have deployed it inconsistently between markets, messaging apps have had inconsistent support for it, different apps have added their own extensions, and interoperability between carriers has been inconsistent. Shit, Google Messages only started supporting it in 2018 with most users needing to use Google's servers to actually get it working across carriers. Google only added E2EE last summer and only in 1:1 conversations between Google Messages users.
So I'll repeat, SMS/MMS is the least common denominator that will reliably work on non-Apple devices and across carriers. Adding RCS support to Messages would mean building their own infrastructure like Google has done in order to have the feature work reliably.
RCS has been around a long time, but it has been deployed by major carriers in just the last year and certainly is not universal. Google also just did a bypass the carriers thing mid last year. It is not a this feature has been missing for years question.
Adding a full feature set is something to approach with caution since Apple does not go the Google approach of burn it down and start over every few messaging apps. Most of the zero days seem iMessage related.
So I'll repeat, SMS/MMS is the least common denominator that will reliably work on non-Apple devices and across carriers. Adding RCS support to Messages would mean building their own infrastructure like Google has done in order to have the feature work reliably.