Because entanglement means some form interaction, connection, or relationship, and only separate entities can interact or connect. Hence, if god would be merely entangled in some way with creation, creation would have to be non-god. And if there's non-god, god cannot be omnipresent. It's the same problem as with immanent god -- that which is permeated by god cannot be god, hence such god is not omnipresent. Which is clearly not the case.