Well, here's the reason: Dirty Movie is all fuck, and no filmmaking.
During the Anal Reverse Cowgirl, she's grinding and groovin' leaving our muscled man falling farther and farther behind.
However, the nature of the relationship with Evan is professional.
Ironically, Stone is the one who started off a dancer, which is the usual career trajectory for women in porn.
The image here is quite cinematic, with appealing framing choices and lots of compositional flair.