Also many times the crews may have timed out and there would not be time to deadhead another one in. Ferry flights generally do not have inflight assigned to them so passengers would not be allowed to ride on them, in fact in a few certain instances even employees couldn't catch a lift on them.

as mentioned above, ferry flights are never loaded into the main reservations system, but they are loaded into the irregular ops system but unless you know the coding to look it up in QIK its hard to find. This is to prevent agents from accidently booking passengers on an unstaffed flight.