They haven’t really. What they really should do is run their own RCS server and federate and support the e2e extension, but they don’t want to.
The most annoying part is that the imessage encryption protocol is so far behind state of art (same underlying encryption protocol with small RSA keys and no deniability since ~2011 when Signal has been around since 2010 with a better protocol). Meanwhile Google based their encryption extension on the Signal production. It would be a solid security improvement if Apple adopted it.