Because in order to use Graphene, you have to financially support the same company that is making Android less free.

At the end of the cycle of a Pixel model, they are heavily discounted. So much that it's probably close to hardware + development cost. Google probably expects to make a profit from Google One subscriptions, Play purchases, and ads.

By installing GrapheneOS, you are giving them nothing.

they're working on it. GrapheneOS has serious plans to get a phone made for them. even more serious now that Google has become openly hostile to their project by no longer publishing the Pixel device tree when new releases come out.

Google could invest in Graphene as a hedge.