They do this so the cum is nearer to the opening, making it easier for the actress to push out, thus providing the camera with the dynamic shot of the cum oozing down. Keep in mind that these actors/actresses aren't having sex for the purpose of their own gratification. They're putting on a performance, more concerned with how it looks than how it feels.
He should have pushed in deep when he ejaculated inside her. What is the deal with guys almost completely pulling out when then cum and just having the tip inside her when they shoot? That doesn’t feel good. He needs to plow her pussy deep.