But why is that? Because after sexual revolution and so called "liberation" of women men have no incentives to be husbands and start family anymore. In general, men will do what it takes to get sex, and sex drive is a huge factor behind met getting married. It is well-known that the traditional (pre 1960s) mating system required men to be reliable husbands and providers in order to marry, and marriage was by large considered the only legitimate sexual release (as it should be). In other words, women and society required men to become husbands and providers in order to get sex and satiate their sex drives.

This was also a moral solution in which sex was restricted to the confines of marriage, as it should be (sure, there were patologies, but I'm talking about the system in general). Also, women married in their early 20s, giving incentives to men to work hard and become candidates for husbands and providers. Fast forward to today. "Liberated" women are not interested in marriage untill their late 20s/early 30s, spending that time at various LTRs/casual sex with whomever they like. Therefore, women no longer require men to marry them and be reliable husbands and providers in order to get sex, so men don't do it, because they don't have to - simple as that.

It has also another layer. Sexual revolution removed all shackes on female sexuality. Women can now financialy support themselves and don't need to lock down a provider husband like they did before the 1960s. All social stigma was removed from fornication, therefore women are free to select their partners solely (or at least mainly) on the basis of sexual attraction. It is well known and proved by research that due to their hypergamy majority of women are sexually attracted only to top 20-30% men (alpha males) - these men get tons of casual sex and don't need to become husbands/providers in order to achieve that.

Other 70% men (mostly betas) receive little to no female attention in their early 20s (women will settle on them only later, when they can't lock down one of the top 20-30% for marriage), therefore they also have no incentives to do work to become reliable providers and husbands. Before the 1960s an average beta male had a good shot at marrying a feminine woman early, as he was in demand as a provider. Now, with "liberated" women supporting themselves and entering the workforce, a beta provider is no longer needed (women often settle for one in their late 20s/early 30s, since there are not enough alpha males to go around, but they are not happy about it, which results in 40% divorce rate).

Add to this terribly unfair divorce laws in the US which are heavily skewed in favor of women, including no fault divorce (70% of divorces are initiated by women - women can blow up marriage for any reason any time and be sure of getting cash and prizes), child support (imputed income, in some states men can go to jail if they lose a job and can't afford child support) and you have further disincentives for men to work to be husbands, again caused by feminism. Finally, husband's authority as head of family has largely been dismantled by the aforementioned unfair divorce laws, feminist propaganda, state education, popular culture, #MeeToo, etc.

Tl;dr - men will do what women and society require them to do in order to be selected as mates. After sexual revolution, advent of feminism and decline of Christianity and its moral values in Western societies, women rejected early marriage en masse, no longer requiring men to be husbands and providers in order to get sex, so men don't do that. Also, the authority of husband and father has been completely destroyed by feminism. Feminism started all of this and remains the main driving force behind this process. All of this is well documented and covered in Christian/Catholic manosphere by such bloggers as Dalrock and Donalgraeme.