There are TONS of reasons not to do "fuck a fan" vids. With an established porn guy, you know his STI tests are current and he's not faking them (or getting tested last week and barebacking a stranger the night before your shoot), you know he's not going to get violent or "accidentally" slip it in the butt if that's not part of the scene, you know he can stay hard, etc. With "fuck a fan" videos, all of that is out of the window. It could go wrong a thousand ways. Even if the dude does follow all the rules you can still end up with a lackluster scene because he was nervous or intimidated.
I love to see real "fuck a fan" videos, but I understand why most girls don't do them.