That would work, but for it to work across different devices/and or carriers, they would all need to work together to implement it in a mutually intelligible way. If it wasn't part of the RCS standard, then it would basically be different device makers and/or carriers having side agreements with each other. So they use their own encryption scheme on either end with RCS in the middle. With situations like that it becomes too easy for the side agreements to not be the same everywhere amongst all parties, which ends up being confusing for consumers and possibly broken. It would lead to things like, oh these features work between these carriers, but not these, or if you message your cousin in Germany, you have to remember to use a different setting than when you message your friend in California, etc. It would be better if end-to-end encryption was part of the RCS standard, which I guess means we already need an RCS 2.0