Basic Premise: Very few women are regarded as "losers". I understand the term is loose and subjective but we all basically have some sense of what a loser is. Almost certainly what comes to your mind when you think of a loser is a man of some age who "failed" at life. Being honest and thinking back to high school, how many girls were regarded as losers or totally undesirable compared to the boys? A guy sitting there by himself at lunch was seen as weird and sometimes as potentially dangerous. A girl on her own will more likely be shown pity and from my experience they rarely existed. After high school things don't get better for men who can't make it socially or financially.

An unemployed man is likely to be thought of as a bum. If he's bad with money, in poverty, or in a lot of debt thats usually a sign of his personal irresponsibility. If a woman doesn't make much money or is unemployed it's not as big a deal. Glass ceilings and the patriarchy might be holding her back. Men don't really have any built in excuses for not working and even with greater careerism amongst women there is not strong expectation that they must succeed in the world of work as proof they are complete people. Women who live in poverty are pitied, not thought of as bums who lack personal responsibility.

If a man is bad with women then that is almost universally seen as his fault and not part of some bigger cultural shift in dating or romance. If a woman is older and unmarried this rarely makes her seen as a loser in the 21st century. In the past it might have been viewed unfavorably but on a personal level I have seen far more people claim the issue with single women isn't the women but rather its men's fault (usually for being losers and not living up to women's standards).

Living with one's parents as an adult is seen as much worse for men. Being a loner with no friends just makes a man look weird. As a rule of thumb people are just less empathetic toward men and they are almost always going to be viewed as more dangerous, buffoonish, awkward, and uncool. Very few women are viewed as nerds or geeks in any negative sense.

Causes: I believe this is mostly caused by two things but I'm open to other ideas.

1. General expectations that men should actualize their potential over their lives. This doesn't mean you have to be a married millionaire by 25 or 30 but it does mean you need to always be doing something. Working, training, in school, volunteer work etc. Neither men nor women really seem to be bothered by women coasting through life if they choose to do so, it doesn't really make them seem lazy or immature.
2. Feminist theories of patriarchy. If men live in a patriarchy then, naturally, we should expect them to live their lives with relative ease, compared to women. graduation, jobs, promotions, raises, marriage, family, friends, and respect should all simply come easier to a highly privileged group. If you cant do then it's your own fault and you must be a loser.

Caveats: I do think some women might be considered losers or there may be some female equivalent to being a loser (being toxic?). But for a woman to truly be a loser she truly has to mess up big time. I'm thinking 4 kids to 4 different dads. Single mothers are sometimes castigated, but even this is mixed as a lot of blame may simply be (deservedly) hurled at the absentee man/men. Habitual cheating, drinking, gold digging and other really bad behavior might make a woman "toxic" but I'm unsure if she would be considered a loser. Still, some women almost celebrate gold diggers tricking rich (losers) into giving up their money.

Another caveat is that some men are genuine losers. I do believe in the moral blameworthiness of able bodied adults in rich nations. Typically, most people are done with college by about age 22 (you start around 18 and take about 4 years to finish where I'm from). The further you are away from college completion age and the worse you are doing the more suspect you are. Failure to get a driver's license, car, move out of your parent's home, hold down a job etc are bad signs. Being in and out of prison, habitual substance abuse etc are all signs of immaturity and progressively less difficult to defend the older a man gets.

I'm pretty generous and am willing to grant a lot of grace to men (and adults more generally) who are making an honest effort and not making excuses when they are at fault. A man doesn't have to be successful with money, women, or getting jacked to prove he is mature and responsible. If he is really trying to do the right thing and admits his faults and flaws, I don't care if he doesn't have much money or can't bench 300 lbs. If housing is super expensive and you're living with parents that is understandable. If you're neurodivergent and can't drive I get it. If you have a glandular problem and struggle with weight loss thats a reasonable explanation.

Conclusions: I think most people have more sympathy for women regardless of where they end up in life. Single mothers might be an exception but there aren't many others. In general we are more likely to unfavorably view men as losers when compared to women in an equivalent situation. If a man ends up homeless, in prison, or at the unemployment office we'll more likely blame him and not society or just give him a pass. If a woman ends up in a bad situation we are much likely to think of her as a victim, not a loser.

I also don't know if there is a solution to this. Being more sympathetic to men might be okay, but it might also legitimize bad behavior and let actual losers off the hook for their bad life choices. It probably wouldn't be good to start thinking of more women as "losers" and it is unlikely anyway.