Pornstars have semi-permanent partners?

[CBGB87]

So I've noticed in a lot of scenes, certain pornstars do a hell of a lot of scenes with the same guy. Why is this?

 

[Hypergalaxy]

A lot of Solo Girls only do their boy friends.. Like Haley Wilde.. Liz Vicious... Raven Riley and so on

 

[D-rock]

It could also be some of them like working a small handful of people they like better, and are not willing to work with just anybody.