Good points all, but the issues are real wrt interoperability. I switched from iPhone to pixel and it was a huge pain in the ass for my friends and family, 90% of whom are iPhone.
They choose not to deliberately to tie people into iPhones.
Apple does do that.
They provide a toggle (defaults to off) not to fall back to SMS. Which makes sense as an option, because it would suck to send what you think is E2E encrypted and then find out it went in clear text.