I don’t think that is the case. I believe it has more to do with production companies that like to hire the same actors. Humtv is family owned. Momina Duraid owns Moomal productions and their productions are solely produced for humTv. Goldbrige media which produced Samdan, Mujhe Sandal kher do also exclusively acts their productions on Humtv. a&b productions and humayon Saeed 7sky produce for all channels. I guess they sell their serial to highest bidder.