For the same reason many good women watch cinema with men being beaten bloody, stabbed, shot & killed- because if it’s a film of any sort produced by a company, action or porn, the impression is that it is not real.
Firstly, I don't think I know anyone who goes to the cinema and wanks while watching because they find the beating, blood, stabbing, shooting and murder scenes sexually thrilling.
Secondly, you are comparing the impression it's 'not real' in a (non porn) movie versus the impression it's 'not real' in porn. In the latter, viewers are well aware that while the people on screen are 'acting' in that their lines are scripted etc, there is a real, living, breathing woman being often violently treated, slapped, choked, spat on, very roughly had sex orally, vaginally and anally. Comparing that with mainstream non porn movies is completely disingenuous.