Their security model is different. They are multiuser OSes protecting users against each other, while Android started as a single user OS (despite what goes on with uids underneath) protecting users from themselves.

Users running their own apps as uid 0 goes directly against the security model. Nothing stops them from changing or completely discarding the security model on their own devices, but once again, that is completely orthogonal to blocking apps on the Play Store.

It's not completely orthogonal, though. Both things are, in practice, means by which Google can control what kind of apps most users will be exposed to and will install.