In real life there's enough time for a girl to get to know how a guy cums and make him blast it out. And to take the time to make him cum. In porn there's very little time and they often don't know each other at all before the scene begins. So jerking off is the only way to get off or it would take too much time and most people on a porn set don't want to be on it longer than they have to.
Finishing with a jack-off is always a huge disappointment...what the lady doesn't get you horny enough to finish the job? When the guy starts stroking his own meat...we always tune out.
Imagine being some young new starlet just broke into the biz maybe to support a bit of a habit. You don't really know of Peter North and his sexual escapades. So one day your agent books you with him. This weirdo old enough to be your grad daddy with a pompadour and mom jeans shows up on set and pulls out a cock thicker than your wrist. And as he's sliding it into your ass and youre wishing you had stayed in school and just as you're thinking you are getting used to that thick cock he slides the remaining 5 inches right up in there and you have 45 minutes to go before he drenches you and leaves you quivering and pondering your career choices.