GrapheneOS provides drastically better usability, robustness, overall functional and app compatibility. Privacy and security are also drastically better in AOSP and especially GrapheneOS than that desktop Linux software stack ported to mobile.

All of the default apps in GrapheneOS are being rapidly overhauled or replaced. It wasn't a priority due to the incredibly good open source app ecosystem with many existing alternatives available. There isn't a similarly large and high quality open source mobile app ecosystem available for what's being promoted.