Otherwise, many parts of online advertising form a very competitive market and if it was valuable (eg you can find customers this way) there would be demand.
It just doesn't work well (yet). Users don't trust porn sites.
People are militant about porn ads appearing on their website.
I've never heard anybody complain about their ads showing up on a porn website.
Then you've clearly never worked in media or advertising.
Most advertisers would be livid if their ad showed up on a porn website. Many of them will even be upset if you put their ad on a slightly risque article.
They don't want their image associated with porn (or tragedy). Even legitimate mainstream news organizations have to put in code to exclude advertisers from certain article categories.
And see eg Private Eye's "Malgorithms" or Need to Know's Doh! gifs for examples of poor ad placement.
Whoever creates a brand ad network of premium advertisers which is tolerant to adult content (or at least to any user generated content) and competes with AdSense will make very significant money.