For the most part the other comments are correct, however in most of these systems the lock has the ability to store the valid keycards and their expected expiry date. This is in case the server or the network go down, the lock will still work.

When a change is made, like a card is lost and replaced, the 'server' will send the lock the new information. So in a way OP is correct in that the locks aren't constantly talking to 'the front desk.'

BTW many office buildings, warehouses etc use the same systems. They just have much longer expiry dates for pemanent employees.