It would be possible to ship generic system images with separate updates for the device support code. However, it would be drastically more complicated to maintain and support due to combinations of different versions and it would cause complications for the hardening done by GrapheneOS.
Sounds like they could, but have resource limitations to do it. It’s also a knock against Google whose hardware has gotten worse. Personally, IDGAF about these project-imposed requirements if I can’t have the standard headphone jack on portable device.