Social pressure. While women have the stigma of being sluts if they have too many sexual partners, men have the reverse. Intense peer pressure to be the alpha male. That having a partner at all times is the natural default state and being single is considered a bad thing. If people see a single woman they will assume that she will have her reasons for being single as if she's completely in control over her situation. Single men will be instantly suspected that there must be something wrong with them, which is why they cannot find a mate.
Of course those males do not feel like it is their fault, so since having a mate is the natural state of things and them being not the problem it has to be something external. Of course these men also lack a good amount of self reflection, intelligence and human decency but social stigma is where it all comes from.
If there was no stigma, people, especially males, would feel a lot less awful about being single and look at things a bit more optimistically.
BTW I do not think Incels and terrorism have a causal connection. It's not like their terrorists because they're Incels. It's just similiar circumstances and character traits that made them both Incel and a terrorist.
If you demand respect or gratitude for your volunteer work, you're doing volunteering wrong.
