Because AOSP is open source and can be built without Google Play running at the background unlike the other alternative: giving up privacy to yet another big tech Apple.

Graphene adds many privacy features on top of regular AOSP. But it only works on phones that has good security features that are not woefully outdated or completely closed-off. Google has complete control over Pixel supply chain and they can make their phones with all bells and whistles for their ends and they behave a bit benevolently and expose the interfaces to the user too.

Most Android phones use Qualcomm which doesn't give a flying fuck about giving control or privacy to the users.