I just want to take a moment to point this out: the hypotheses people like advancedatheist push for why they're incel are very emotionally salient (a small number of men are monopolizing all of our women! omg!) So everyone, please don't let this very emotionally salient hypothesis prematurely crowd out other explanations for the same phenomenon.

Stanford psychologist Philip Zimbardo wrote a book called the Demise of Guys. Among other things, he discusses the sexual frustrations of modern men and offers some possible explanations:

In [the 70s and 80s], about 40 percent of a large population of Americans described themselves as “dispositionally shy”... However, since then the percentage of those reporting being shy has steadily increased up to 60 percent. That rise has been correlated with increased use of technology, which minimizes direct, face-to-face social interaction. It also reduces social practice time and learning the many rules of constructive social dialoguing.

...what is different today is that shyness among young men is less about a fear of rejection and more about fundamental social awkwardness — not knowing what to do, when, where or how. At least guys used to know how to dance. Now they don’t even know where to look for common ground, and they wander about the social landscape like tourists in a foreign land unable to ask for directions. They don’t know the language of face contact, the nonverbal and verbal set of rules that enable you to comfortably talk with and listen to somebody else and get them to respond back in kind. This lack of social interaction skills surfaces most especially with desirable girls and women. The absence of such critical social skills, essential to navigating intimate social situations, encourages a strategy of retreat, going fail-safe. Girls equal likely failure; safe equals the retreat into online and fantasy worlds that, with regular practice, become ever more familiar, predictable and, in the case of video gaming, more controllable.

He's also got a section on how men are being diagnosed with erectile dysfunction at younger and younger ages, linking to the site yourbrainonporn.com which discusses this.

Are we really supposed to believe that evolutionary factors like female hypergamy are responsible for increased shyness and erectile dysfunction among young men? Female hypergamy, insofar as it exists, is a mostly static biological phenomenon that's been around for 100s or 1000s of years. Are we really supposed to believe that right around the time when the world is changing faster than ever, suddenly female hypergamy goes from being a constant in the background to a destroyer of societies? I'm sure the liberation of women plays an important role here, but I think its role is frequently overstated. Think back to the 60s and 70s when the sexual revolution first happened. Where were the hopeless incels back then? Or think of forager societies where chastity was not held to be valuable... where were the "omega males" at that point?

Anyway, yourbrainonporn.com also has a page on how excessive porn use may destroy social confidence. Like most addictions, porn decreases your brain's dopamine receptor levels, and lower dopamine receptor levels have been shown to predict lower social status in monkeys. Anecdotally if I avoid porn completely for extended periods my social confidence and abilities with women improve significantly.

(This also matches perfectly with nerds being worse with women if they spend more time alone with their computers.)