Simple - feminism and women's empowerment. For any woman lucky to be born in a Western country, the world is her playground.
How?
Before feminism, very few people remained single, and in traditional societies, young people are paired up by their seniors (either arranged marriages or indirectly). Parents want grandchildren because they want descendants. Traditionalist societies, despite being seen as 'stupid' and 'backward' by the West, are wise enough to know that a surplus of single, sexually inactive men is dangerous for society.
With female empowerment, more options in dating came along. Think of it like this: give a 10 year old freedom from his parents. What is he going to do? Stay up playing video games until 2 AM, and eat brownies for dinner & ice cream for breakfast. It's similar with women: No longer having to listen to their pushy parents and access to birth control, they could go out and have fun with the hottest guy they can find. The dutiful, hard working engineering major is no longer a catch; rather, 19 year old women are hoping to hook up with the most exciting guy they can come across, and in this process, they ride the cock carousel until they find the right guy when they're 32, only to divorce him 2 years later. Their (non-Chad) male counterparts, in the meantime, are getting rejected constantly and deal with merciless dating market. Western men that didnt luck out genetically in have two options: become thirsty simps, putting up with shallow female behavior, or 2) remain virgins until their mid-20's and beyond.
Guys like me, that lost the genetic lottery and live in Western societies , do not have the cushion of getting set up with a girl. We become incels.
PS - Yes I'm aware I compared women to 10 year-olds