I've pondered this question often over the years, and various thoughts come to mind. Speaking only about the USA, my country is really restrictive on public nudity, and in most places you'd be thrown in prison if you go for a "nature walk". Because the society I live in has made nudity a forbidden fruit of the sorts, alot of people develop a warped perception on the topic, and their resulting attitude on it morphs into obsession. If there were no controversy on public nudity, I rather doubt there would be a big deal about pornography.

It's better than going to church ! [ come all ye faithful ! ]

Why do people watch cooking shows?
Personally I don't watch either.
The meals prepared and the women in the movies are all out of reach for me.

Because is Free?

It separates the person from a sexual encounter and there is safety in that distance. It enables the viewer or reader to escape the shame, the sin vs virtue morality associated with an actual sexual encounter. The victims in literary or visual porn are not hurt, they come back for more and more, similar to De Sade's victims.
Emotions in porn are typically simple and direct. When they are not, such as in Anias Nin , DH Lawrence, Henry Miller or the Cocky Boys, and the performance art of Marina Abramović et al it become erotica.

Because people don't normally get the women or man they want, so they are left to please themselves. Males get off by sight more than females do.

I believe it is popular because it is easy to NOT think while watching it. It's easy to not think of your dignity. It's easy to not think about the feelings of your significant other. However it is easy to indulge in your self gratification. I don't care for the stuff myself but I get why some would.

You're kidding!