Sex work is mostly unregulated and is prone to disease, crime and violence. 

To work the upscale market, a lo-icnome man has to have money to invest in clothes, beauty products and other high end products.  The wife has to be beautiful, discplined and spends time and money to exercise, and groom..

Given all of these barriers, if they can make it to the upscale market, with all that discpline, motivation, investment, planning, they can make money in any other areas.  There is a reason why they are working in McDonalds.