I think a lot of it is because it makes writing really easy. You don't need to come up with a reason for these two people to be near each other in an intimate setting, which they can easily turn into something more sexual. Not many people are watching for the plot, so they just need a bare bones contrivance for it taking place.

You never quit beating your meat mid-fap because the plot was just so captivating?

There's sex with some level of plot, and there's plot with some level of sex. It depends on which goal is greater for that movie or book. There is also a variety of degrees of porn plot, from bad to decent (with still the goal of being porn, but far more believable).

Are not-step-siblings still okay? Short of being admitted in the script, how does one know the relations? I guess the same validation as when the actress says she's 18, right?

I think because in order to product the kink you need to set a ground base for story. A lot of people just want some sort of story instead of just straight into it.

because actual parental incest would fire up America's family values.

Families have inherent relationships built that can be nodded at without much conversation being necessary. Basically porn with plot without requiring as much plot.

There is a specific group that is in control of most of the porn websites. Lets just say they are super into it. For reasons.