That's what I used to think too, mostly because our society now normalises porn, but what they are doing is thoroughly part of prostitution.
They not fake having sex on camera. They are having actual sex with a stranger which they're usually doing for money. The money they get may not be coming from a single audience member, but that money is coming from somewhere.
Let's say someone hired two people to have sex in front of them in a hotel room, for them to watch masturbate to. Maybe they partly paid for that, or maybe their mates paid for it, would you say that isn't prostitution, just because they're watching? The only difference here is that there is also a camera there and it's being broadcasted to thousands, sometimes millions of people who are either pay or don't pay.
People think of it as acting partly because they associate it with studio settings. It would be quite a weak argument to make because today, with the presence of the internet in everybody's lives and the ease with which people can make material themselves, porn is no longer just occurring in studio settings in front of professional cameras. It's happening in that hotel room and being broadcasted.
It's also irrelevant if it's happening in a studio or not - it's still a commercial activity. Whether or not they're doing that in front of a camera in someone's bedroom, or doing it in professional type studios, there is money being exchanged somewhere and that is prostitution.