Would that be because maybe sexual attraction to a person is based on the personality we project onto the looks and energy? And maybe within an industry you simply don't have every type of personality and that is actually really hard to fake?

Which is why some people like the amateur stuff much more.

Of course this suggests there are deep layers of emotional needs behind our sexual preferences. Which is obviously not true. It is all about the flesh. That is why porn sites only need three videos ;)