The fact that the neighbours tend not to know of their lycanthropy is simply a matter of safety.
Were-cormorants usually fish in small groups, where one stays in human form and several others (2 to 5) use their cormorant form to catch fish. The human form were-cormorant is often an elder whose reaction is already a bit too slow for fishing, but who has much expertise and can advise the others. They also steer the raft they use to transport the catch and might even do some preliminary cleaning.
Were-cormorants generally just wish to be left in peace and would prefer that their neighbours not find out about their lycanthropic status. This sometimes ends up netting them into doing scouting work for outsiders who find out about their secret.