There is no one specific answer. I would say a top drawer reason is the scarcity effect. Sex is prewired into peoples' brains and the hormones that come with puberty are all designed to essentially start that process of desire and sexual fulfilment.

While it's not the only agitator, feminism in it's guise in the early 2000s meant more young women had to prioritise their careers and education. This isn't a bad thing in and of itself, the key thing here is "had to". In order to afford to live in a modern society, together with the messages being handed down by news media and eventually social media that men were the reason everything was bad, that meant less women had any desire to be involved in a big relationship with a man.

Of course, men have in recent history already become accustomed and raised to further education or to get out to work and to essentially take responsibility. But because of that upbringing, men are (or were) adept at being able to foster their loving relationships while doing that. Women on the other hand were given the message that it was one thing or the other; you could not be in a loving relationship AND focus on your education or career aspirations (which had better be lofty, by the way). This isn't me saying women are all bad, this is me saying women and particularly young women were put under enormous societal pressure to do this thing or be worthless by a huge mega consortium of news outlets and constant messages.

So let's get back to the men. There were less women willing to engage in long term relationships. Of course, women were happy to engage in brief sexual relationships or one night stands (as is their agency to do so of course) just as men had done. Again, this was something promoted by feminism. But a good majority of men were basically not able to get involved with women, were not engaging in sex with girls their own ages or even older or had to settle for someone who was not exactly right for them as they were willing to engage.

Going a little psychological here, what is it that men like about women? I mean there are hundreds of answers, but essentially men are a little gruff, a little rough, women are soft and curvy and have boobs that make us crazy. Women have the attributes about their bodies that men don't. And men like them.

Thus when men are for want of a better word denied what they crave, that's when the mind attempts to find that in another way. If you can't get girls, why not be a girl? The logic is sound regardless of whether the execution is there. If you want to see a cute girl in her panties but cannot, don the panties yourself and convince yourself that it's all good.

OK I have had to be brief as need to get going and this really is not the only reason. This is just one avenue that has led us to the society we have today of men attempting to become women. And selling it to children wholesale is just a method of legitimising it.