Yes, because according to the GrapheneOS devs it would weaken the security properties of the OS.

Besides, GrapheneOS already supports the native Google Play Services (with additional sandboxing), so I have a hard time seeing how MicroG would help here. (I used MicroG on LineageOS for years and it certainly wasn't without bugs.)

Also, regarding what GP wrote, I haven't encountered a single app in a long time that wouldn't run on GrapheneOS.