not everyone is desirable, that's just a harsh fact that incels need to accept.
Really? I always thought everyone - or at least almost everyone - was desirable to someone. Most people are desirable to numerous other people. Failure to recognize attraction, refusal to risk interpreting attraction signals for what they are, fear of rejection manifesting as suppressed response to such signals etc., all have a role in rationalizing anger/frustration at their own failure to satisfy or at least tame their own primal sexual urges.
I know a lot of short ugly men who do just fine with “the ladies”, if for no other reason than that they’re fearless. I envy that quality, but despite lacking it I have not ever had any basis for complaint about lack of availability or opportunity for relationships and/or sex. And it’s not because of my movie star looks or vast fortune.
I'd guess, for men, it's usually about intellect
I suspect that emotional immaturity/insecurity has plenty to do with it.