They offer a specific experience/product. As men we like to push the envelope and see what else can be had---things not specifically on the menu.
Natalia gave me one of the best massages ever, with that satisfying conclusion that we pay dearly for. But that was all that I wanted, and that day I wanted her to do all of the work...and she was a great conversationalist.
But I prefer FS in clean apartments with Kgals, not funky studios in the Tenderloin. Not at that price.