Why did the show recycle so many actors

[TMC]

TV Tropes lists this particular casting trick or habit under their "You Look Familiar" page. I don't think that I have time right now to list all of the specific occurrences on The Partridge Family reusing actors.

But today, I was watching the episode "A Penny for His Thoughts" from Season 3. The episode featured Stuart Margolin (AKA Angel Martin on The Rockford Files) as Snake. I got confused for a moment because Margolin had previously been in the Season 1 episode "Go Directly to Jail", so I was wondering at first if he was supposed to be the same person. And then I remembered that Margolin's character's name in the earlier episode was Hank. And Margolin in the latter episode was actually supposed to be the same Snake that Rob Reiner played in the Season 2 episode "A Man Called Snake".

There was another Season 2 episode called "All's War In Love and Fairs" that guest starred Harry Morgan (AKA Colonel Sherman Potter on M*A*S*H) after previously appearing as a different character in the Season 1 episode "The Sound of Money".

Also in Season 2, Arte Johnson, who was on Laugh-In appeared in the episode "My Heart Belongs to a Two-Car Garage" as a Russian painter. And then in the next season, Johnson appears as an escaped convict in the episode "For Whom The Bell Tolls..And Tolls..And Tolls".