>>23383709There are three big reasons.
1) There simply are a lot more men than women. This is a global truth written into our genes, it varies but men are always born at higher rates. It was useful in the past cause many of them died in wars/defending from animals/exploring new possibilities, but now the world is just too safe.
2)Since women look for men with resouces, they have a natural tendency to pair up with older men who have more resources. This leaves younger guys with less options, but with hoper for the future. However now you have, especially in first world countries, this moral panic about dating younger girls. This is partly caused by the first issue, but as things progress is a feedback loop that only worsens things.
3) It's the point that incels usually make about desirable men dating multiple women. In reality it's actually the least influential among these problems, but it is still a reality. Women want to date up, and men if given the chance are very willing to go for as many girls as they can. This was always true but is exacerbated by our culture of comparisons.
To all of this though let me add: rates of loneliness and singlehood have actually greatly increased among women too, though at much lower rates. However both the lonely men and women who can't get relationships end up falling into a spiral of learned helplessness, depression and resentment and become more and more unattractive, perpetuating and worsening the cycle.
Basically we're fucked.