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 high quality open source mobile app ecosystem available there.
“rapidly” - how long has it been?
We hired 3 experienced app developers a few months ago. That's now our app development team working on overhauling these apps. They've already replaced the entire Messaging app user interface with a new modern Compose UI. Messaging v13 is currently in the Alpha channel and v14 is on the way with a bunch of additional fixes needed for it to reach Beta and Stable. Multiple other apps including Contacts are well into the same overhaul process.