Why contractual terms forbid to shot for all LP-studios?

Can someone please answer this question?
Does it make sense from the most perspectives, especially the economic perspective? Because in the end it is all about money...
Assumption:
A fan likes a pornstar and want to buy as many great scenes with her as possible. He/She doesn´t care about the studio/director, just the quality in total is important, that means great camera work, sex-positions, outfits, performance of male actors.
1. Problem - from the view of the fan:
Not every scene of one studio with the pornstar is great. And not every studio makes great scenes.
Fan´s behavior:
The fan buys only the good ones. He/she doesn´t care whether the girl shots exclusivly for "the" studio. He/she would buy more, but the pornstar is not allowed to shot for other studios, which might be able to shot great scenes.
2. Problem - from the view of the pornstar:
The pornstar has fewer scenes. The probability to be in scenes which makes her famous and bring her more scenes is less. The income is lower and there is no stability. The risk to be out of the business after only a few scenes is higher.
3. Problem - from the view of the boss:
Potential successful scenes are not shot. The revenue in total is less.
Studio´s potential motivation:
Shooting a pornstar exclusivly to increase the image, the revenue and attract more customers. But the problem is the fan´s behavior above.
Does it make sense from the most perspectives, especially the economic perspective? Because in the end it is all about money...
Assumption:
A fan likes a pornstar and want to buy as many great scenes with her as possible. He/She doesn´t care about the studio/director, just the quality in total is important, that means great camera work, sex-positions, outfits, performance of male actors.
1. Problem - from the view of the fan:
Not every scene of one studio with the pornstar is great. And not every studio makes great scenes.
Fan´s behavior:
The fan buys only the good ones. He/she doesn´t care whether the girl shots exclusivly for "the" studio. He/she would buy more, but the pornstar is not allowed to shot for other studios, which might be able to shot great scenes.
2. Problem - from the view of the pornstar:
The pornstar has fewer scenes. The probability to be in scenes which makes her famous and bring her more scenes is less. The income is lower and there is no stability. The risk to be out of the business after only a few scenes is higher.
3. Problem - from the view of the boss:
Potential successful scenes are not shot. The revenue in total is less.
Studio´s potential motivation:
Shooting a pornstar exclusivly to increase the image, the revenue and attract more customers. But the problem is the fan´s behavior above.