I’d think the actors had a contract with the studios and this was the a way for the studios to fulfill this contract and give them them airtime the were due or get the most out of them (probably more likely). I’ve seen on old episodes of Culumbo, for example, George hamilton !V played at least 2 different episodes, as did a few other actors. Maybe in those days their appeal was strong enough to get away with re-casting them? It’s a good question.