That's what incels with defective personalities tell themselves. It's also not true. Just look around you ad see what men out there are actually wandering around with partners. Or, look at it another way. There are plenty of married slashdotters, these days. Either slashdot has a proliferation of lantern jawed musclemen or there's something up with your assumption.
If it's all in people's heads why is there way more sexually inactive males 18-24 than before, without a similar increase in females of the same age group? It is well known that the average age of first child birth has been going up, for many their late teens and college years are casual hookup and FWB years and they don't start looking for a life partner until their late 20s so they can have children around 30ish. Obviously then they have to match up one-to-one so it becomes a run-off lottery, these men are already taken so pick from the remaining pool. Now that's an oversimplification, but it explains why it evens out as people get older.
The same is not at all true for just having sex. Tinder has made males be almost a living sex toy catalog and with so many to choose from why settle for anything less than the best? It's ridiculously superficial but it's become more like going to a restaurant, you have some places you keep coming back to (FWBs), some you occasionally go to (booty calls) and some to experiment with (Tinder). But you'd never really need to go to the shabby pub to try out their food on the first place. Maybe it has charm. Maybe the food is made with lots of love and attention. But if the exterior and first impression isn't great people are already walking (swiping) past you.
Of course some people are just toxic from the beginning, but it seems like the transparency has made a very fuzzy market where most people would get lucky if they just tried long enough through a combination of persistence, timing and intoxication into a much more objective market where some people get the dates and others don't, with a much cleaner division than before. And then of course that inexperience and rejection grows into even more unhealthy tendencies. Then people try to convert these into life partners and find either players who are used to more than one pussy on one end and incels who's had none for too long on the other.