The way the internet contributes to the problem is that it makes it much easier for violent extremists to connect to each other and together they amplify and reinforce each other's extremism.

As to why incels think they're not getting laid: people are very good at lying to themselves, and rarely have the strength or honesty to face negative aspects of their own personalities. It's much easier to lash out at others and blame them for problems of one's own making.

This is seen again and again in all sorts of abusers. Self-identified incels are just a new version of this.