In some cases the person might be very talented at doing clothed kink or fetish shows. I've done plenty of shows where I kept my clothes on by request, but the things I said were filthy and would never be acceptable on YouTube or twitch.
Also, simply being on a sex site gives your viewers the impression that thinking of you in a sexual way is permissible. I realize that some men could have sexual thoughts about a tree stump or door handle, and some men give zero shits about whether their thoughts are welcome. But I think there's a good portion of the male population that feel a little guilty in their hearts for thinking dirty things about girls on YouTube, who are just there showing off their vanilla talents. A model on a cam site doing the exact same talent implies that s/he knows you might have adult thoughts and jerk off, and that's ok. Adding that undertone to a clothed talent show might work a lot better for some models.