GrapheneOS's approach is Sandboxed Google Play so the apps don't get special permissions. Without privileged access, Google apps get access to nothing more than what other apps can get access to on Android. So, for example, even if a device has microG installed, an app with a Google library would have virtually the same access as if Sandboxed Google Play was installed. And why would anyone need microG if not to provide compatibility for apps that rely on Play Services, aka apps with Google libraries?

Also, for some reason some people think that microG means deGoogling. That's not at all the case. microG still communicates with Google servers.

microG doesn't meet the project's requirements. If you look at the big picture, microG offers worse app compatibility and is less secure. The cons outweigh the pros by a lot. "Google bad" isn't an argument that works for us. App compatibility is very important.