Massage Parlors in Houston

I frequently travel along the street FM2920 to get to and from my job in North Houston. I have noticed a rise in massage/spa centers over the past five years; it seems like there are now massage parlors in every shopping center.

Has anyone noticed anything in their neighborhood recently? Or does anyone know why that is the case—is it a front for illegal activities, is it easy to run, or is it profitable business?