"AOSP needs a reference target that is flexible, configurable, and affordable — independent of any particular hardware, including those from Google." [0]
Emphasis on independent of any particular hardware.
Current speculation/inference suggests it is because of the antitrust case against them, preparing for the possibility that they may be divested of Android (or at least to decouple in meaningful ways [1]).
[0]: https://www.androidauthority.com/google-not-killing-aosp-356...
[1]: https://www.bloomberg.com/news/articles/2024-11-18/doj-will-...