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.
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".
[1]: https://www.reddit.com/r/apple/comments/1xuzif/what_ever_hap... [2]: https://www.engadget.com/2017/10/16/apple-faces-440-million-...