Recently I thought about the generational dimension of this whole incel thing, namely the arrival of the internet and the supplanting of an actual sex life by porn, limitless porn.
In porn, often at least, there is no "work" to be done to enter the sexual experience - it just happens as if, say, it was written that way.
I tend to think this has deep psychological effects on teenagers (I have boys in mind, but maybe there are similar phenomena for girls, femcels, etc.), namely that their libido gets sequestered into the virtual, wherein satisfaction is abundantly available and requires no work to achieve.
This may establish a standard in the mind of the teenager, who now thinks sex ought to be as free and simple as it is in porn. The real life complications don't add to the porn, so they are omitted.
Naturally, a resentment begins to brew in the hearts of those who have been exposed to so much porn, and have had their standards skewed to such a degree. (edit: that is, they feel as if their real world would-be sexual partners are too picky, cold - as if they are being denied something they have learned via porn that they deserve so freely)
At least, that would be the conclusion drawn from this hypothesis: that omnipresent porn played a role in causing the psycho-social phenomenon of incels.