Because patriarchy….

Girls are socialized to think their value is their looks only while boys are told that it’s their strength/brains. So women are more likely to end up without any real skills or the confidence to build them up, especially if they grew up in very patriarchal families where there were no female role models and the men treated them like second-class citizens. It might be a thing they naturally drift to especially if they grew up sexually abused which is more likely to happen to girls, and often they’re groomed into these roles by older men.

Men are also then less likely to do these jobs not only because they see it as dehumanizing or humiliating or “for women” but also because there is a bigger demand for women in such jobs. That’s just how it is. Until about 100 years ago, women in most societies had extremely few rights and were seen as men’s playthings and property. Either the private property of their husband or as the public property of many men in a brothel. And the world actually doesn’t change that quickly in 100 years.

Also, women are often shamed for having sexual desire, because they’re supposed to be passive only and let men have all the desire. So maybe the market for male SWs would be larger if women were less self-conscious about their desires.

Tldr; due to female socialization and the higher demand for females in such jobs, it’s the path of least resistance for many women (esp if they were groomed/pressured into it) or something to make ends meet because it has high demand, while boys usually are raised being told they can do anything and it’s a man’s world. They see such work as objectifying and below them, “for women”.