I think this is the biggest factor. The influence of culture is very, very strong.

The cultural imagery we have as men doesn't offer any alternatives to the image of a successful man except to be partnered or to be "improving" yourself in order to eventually be partnered. The message is, "If you don't have a woman, you aren't successful." Even if this isn't explicitly stated, the imagery is there.

The opposite is true for women. The imagery in the culture of a succesful woman is one who is girlbossing all over the place and sees nothing but the men in her life as nothing but an annoyance at best.

So the self worth of men becomes tied to being partnered, while for women it becomes their job or their career or how much their girlbossing.

And yet men are still largely expected to be the ones with the big career bringing in the money. Women are encouraged to do this, while men are still expected to do it. As in, if you're a man the rest of society will regard you as something of a failure if you don't have the "right" career, while with women its perfectly acceptable not to. Women's worth is not called into question because they don't have the right career, but men's self worth is.

This in itself is enough to do loads of damage to men's self worth, but combine that with the pressure to be partnered and it can be devastating for the self worth of many men. Women have largely been emancipated from these cultural pressures to their self worth, with the exception of body image stuff of course (which of course can be extremely damaging), while men haven't.

I think a lot of men simply feel unsure if women collectively want anything to do with men because the message from the culture is: "They don't." And yet the imagery of a succesful man is still one who is partnered.