The primary reason is economic. Photographers feel they invest a lot of time and creative energy in getting experience, and setting the camera, pose, lighting, background, and extra shots to get the right one. They generally price their services by taking into account the fact customers will purchase their prints from the photographer. Thus, the photographer wants the customer to come to him or her to request reprints so an appropriate fee can be charged. They may authorize the customer to have prints made anywhere.
Some photographers also are concerned about artistic integrity.
Since their name is associated with the photos, they want control over how the reprints look. There may be many other reasons. You are encouraged to discuss these issues with your photographer. That way, his or her position can be fully explained, and you can obtain the additional copies you desire. If we cannot make the copies for you, go to your photographer and request them.