The bundling policy is explained here:
There are drawbacks to bundling apps into the OS and few advantages in most cases. Rather than GrapheneOS bundling a bunch of apps, it makes far more sense for users to install their preferred apps from the developers if they can self-update, from the Play Store (via sandboxed Google Play or Aurora Store), F-Droid, Amazon Appstore or other sources. GrapheneOS also has a first party app update system for a first party repository with higher robustness and security than the existing options. Rather than bundling apps, it could just offer recommendations as part of an initial setup wizard. Users have unique needs and preferences and there has to be a very compelling reason to bundle additional apps with the OS.
You can install Play Store from GrapheneOS aforementioned first party repository if you don't like Aurora. It's currently the most secure and reliable way to install and update apps.