Because women typically have different mating strategy -- hypergamy, which goal is to decrease the mating pool size as much as possible down to the best candidate. Men's mating strategy is polygamy which is increasing the pool size as much as possible.

In other words most women would prefer to stay alone forever rather than marry a man who they consider suboptimal.

Add to the equation the "mating standards" that are overblown by Hollywood and social media and the picture of dating market becomes even darker.

I believe this exact mechanism (also aggravated by social media) is responsible for the decline of fertility rate below the reproduction level in countries with well educated population.