Sex is reciprocal and you are not entitled to anything from another person.
Calling it "incel" and making it a phenomenon is a very convenient way to place the blame on women.*
(Envy of men having all the sex is part of the misogyny, not really that nuanced.)
But I certainly see some sociological factors, like women gaining in on men in terms of education, income, social/financial independence, and more people living alone (in the developed world). With women preferring dominant men, including socially and financially the balance has shifted. That's a long discussion.
It's funny you mention online dating and how men get less sex (do you see causality?). I instinctively think the opposite: it's much easier to find a potential partner.
* "Incels" are only men, as I understand it.