Some of the smaller studios have made videos with only internal cumshots, such as the
Pulling Out is for Porn series. But these often tend to be small producers, often with less than professional level models.
But by and large, you are correct that porn, gay and straight, almost always ends with a big cock being jerked to completion. I think the producers have several reasons for staging this so-called money shot. For many guys, cumming is the most exciting and gratifying part of sex, particularly if they are watching porn instead of fucking for themselves. So, they want to show the load in all its glory...how much, how hard and far it is shot, how big a mess it makes, etc. Also, the viewer is probably stroking his own cock while watching, so he can relate to stroking a load out, and hopefully blows his own at the conclusion of the scene, since cumming is often contagious.
Some reasons also have to do with how studio porn is sometimes shot, which is not just recording natural sex. I remember the last porn shoot I did (I'm an old man now...), which was shot in a completely non-linear sequence, with about 2 hours of raw video needed to make the 35 minute final scene. After fucking for over an hour in the various requested positions (without cumming), we had to recreate many of the positions again so better still shots could be taken, without interfering with the primary video recording. Only then was I told I should cum, but the build-up to the magic moment was long gone. So we fucked a little, pulled out, and I stroked out the load. That actually took a lot longer stroking than it should have, but thanks to editing no one sees the delay. It wasn't that the bottom was bad, it's just that after being told to hold off cumming for a long time, it can be difficult to do so when they finally need the load.
