MicroG requires signature spoofing, which Graphene deliberately does not support. It is more secure to run the real Play services in a sandbox that forces it to be a userland app than to run MicroG as a privileged system app with spoofing.

That’s because of a difference on protocol (iMessage vs SMS). This wouldn’t matter if they chose to support RCS which is effectively the Android iMessage equivalent and is an open standard (on paper, not necessarily in practice) but that will never happen.

Look into the app Repainter. It isn’t free and needs root or Shizuku access but does the trick.