It isn’t, though. Group MMSes are notoriously fickle, and doing something like sending a video to the group without it appearing in potato quality feels all but impossible at times. iMessage simply is better for group messaging.
[1]: https://www.reddit.com/r/apple/comments/1xuzif/what_ever_hap... [2]: https://www.engadget.com/2017/10/16/apple-faces-440-million-...
It tough to blame Apple for not open-sourcing a protocol they are sued about by a patent troll. Until resolved properly this would be a toxic code-base anyway.
Of course they could restart from scratch, but I guess they have no incentive to make efforts about that. What will be meaningful however is be what they'll do after patent exhaustion date of infringed VirnetX "patents".