Because Google wants Apple to adopt RCS in order to prevent Android users in the United States from switching to iOS due to green/blue bubble social pressure.
I don't care what Google wants, I care about an open standard and ending vendor lockin. Did you think it was a problem when Microsoft refused to release MS Office for Apple as well, many years ago?
I use Google Docs exclusively. Sometimes I'll use Apple's built in document apps for local.
Microsoft probably makes more money having Office on Macs than not having it.
Office software competition would have built up from Macs not having MS Office. And MS Office would have stronger competition than today.