Doing scenes on OF is a completely different world from shooting in the industry. You have more control over your partners, over the setting, over the tone and pace of the scene, over the positions, over when you wanna fuck and how much you wanna fuck... over everything. I mean, I just recently watched the scene that made Jessie Rogers quit the industry. It was one of those Kink dot com super rough 5-man gangbangs. Some of those dudes looked like they were literally trying to murder her! Banging her head on the hardwood floor... lifting her off the ground by her neck like the Terminator, choking her out really viciousy.... There's a lot of real pain porn chicks go through.
And even if we're not talking about the explicitly ultraviolent stuff, just some of the positions that the girls have to assume in order to make it look good for the camera are kinda unnatural and extremely uncomfortable. And you gotta hold those positions for a long time while the director moves the camera around... it's not easy.