I suppose because sexual gratification is one of our species basest instincts, and due to the way our brain evolved to be social creatures, we have mirror neurons[1] which fire when we observe actions as well as when we perform them. For this reason, viewing sex can emulate (to an extent) our primal desire to mate.

Everyone has different levels of libedo however, so it's likely that your lack of understanding is due to your own lower libedo not generating the same level of sexual desire. Or, you are the kind of person where the emotional connection involved in sexual relations is far more important, which obviously means watching people you have no connection to would not arouse you as much. Alternatively, it could be that you may have decent access to actual sex, which is obviously more gratifying as a general rule.

[1]: https://en.m.wikipedia.org/wiki/Mirror_neuron

Very good answer I must say

This is the first time I've seen libido written as libedo. Could it be a typo?

you might be amazed at how little actual content is consumed, despite the high traffic number.

these sites get a lot of traffic(clicks and page views) and serve a lot of bandwidth(videos) but most users are just clicking around and watching just a few seconds to a couple minutes of any video they load(due to skipping). I think a lot of the traffic just people looking for the perfect scene or something new... there is a lot of junk on those sites these days.

source: i used to manage a number of high profile adult websites.

Why do people like alcohol or any other vice that some group may frown upon? I don’t think you’ll find a satisfying answer here. Different people like different things.

Keeping your prolactine levels high keeps your mind clear.

Evolutionary reasons are pretty much self-evident, I think.