It still boils down to economics. If you have no education, no skills and no money, it's one of the easier ways to start earning. If you're a gal you can even just fake your orgasm and your customer would know no better.

Though it's not always about being totally destitute. I remember reading a book were they interviewed a woman who was taking her master's I believe. Working as an escort meant she can earn money for far less the time her colleagues need to work regular jobs, meaning more time left for her studies and other project. She could have gotten her degree flipping burgers but she figured out that her time was more valuable.