The OP's question brought up something that I never thought to ask. What makes the AI create new Bishoprics? At the rate the AI revokes titles and captures provinces, most Bishoprics (and Arch-Bishoprics) should disappear over time, but there's always plenty out there.
My wild guess was that if the AI doesn't have any spare adult sons and he needs to create a new Count (demesne limit reached), he randomly picks a courtier in his court (mother, underage child, pot-scrubber). If that random courtier is an adult/male/unmarried with finished Ecclesiastical education, he automatically makes it a Bishopric instead of a regular Count? Or does the game say 'We need a certain % of these types', or this province is always a Bishopric - IE if the AI annexes it, the game makes him make it a Bishopric again? Anyone with Theories or Knowledge out there?