I think the simple reason is that pro porn is what it is - fake. As hot as some of the girls might be and as impressive as big loads like Peter North's are, it's still a woman doing it because she is getting paid.
Amateur porn is so great because it's real. It's a turn on to know there are girls in the world that will do what we love (facials

) because they either naturally like it or care about us enough to do it.
I've got a modest porn collection, but I have to say seeing past vids of my wife taking a load down her throat or across her face is way hotter than the best pro facial.