Could be they don't want devices to have latest Android before their own latest flagship devices are released. They might release the AOSP in Aug when the new Pixel hardware drops.
When you say that they dont want the latest android before their own flagship devices are released, do you mean people running AOSP before the new pixel releases with it or other phone manufacturers/carriers beating them to releasing it?
If the second one, that seems a bit unlikely. Atleast here, I don't think I've seen a carrier that delivers an updated android without taking a long time first - iirc this was part of the reason google made it possible to split out security updates and such, to kind of work around that
