That we're talking about actors pretending is understood, otherwise the question of rape/incest porn would already include the actual criminal acts of rape/incest themselves.
But as I have shown above by citing the German criminal code, rape porn (i. e. actors acting out a fictional rape scene with hardcore pornographic elements) is illegal in Germany, and I am sure other places, too, and while I cannot cite a specific law from any country, I am pretty sure that incest porn is illegal somewhere, too. As I said, if you watch videos you will sometimes get disclaimers with a content warning and that you shouldn't if it is illegal in your jurisdiction. The reason for that is that you don't want to make people act out what they saw on TV, which is a thing that unfortunately happens.
That rape does happen in mainstream non-porn (but often nudity including) movies - like the critically acclaimed "The Accused" starring Jodie Foster - is true, however (a) it is not porn which makes legislation differ, and (b) they are usually not depicted to make people wank their dick, which also is a different story for many aspects.
Legislation in general can already be very different, with a sneered upon topic as porn it easily becomes more convoluted, then you have regions where you have very orthodox religious views (I am not sure but I think it was Lithuania, where clergy in porn is a problem - not outright forbidden but some stuff has to be made sure), and you have laws which seem completely off when you're from some other place, like Japan, with tentacles, loli and rape being a-ok, even in that combination, but please pixellate the genitalia. Or like the US, which has probably the most encompassing "free speech" laws for this kind of movies yet suddenly gets female nipple panic in places.
This might not be consistent, and what of that makes sense can be argued (i. e. many legislations, including Germany, also see drawings of children as child pornography) but the fact is that your assumption is wrong, these kinds of porn are illegal in major jurisdictions. Since you seem to be from the US: in the struggle between active freedoms (i. e. you can do what you want) and passive freedoms (i. e. you have the right to be left alone by others), the US is probably the country leaning most towards the active ones, whereas (almost?) every other country will curtail your active freedoms more to protect other people from you. Of course this is strongly oversimplified.
But TL;DR - this has a lot to do with legislation in many places.
