Asian, European and Latin women have economic and security incentives to escort/massage in America. American women not as much. Every foreign country is poorer than America and more difficult to make money. Every reasonably decent looking woman from a foreign country can figure out a temp visa, how to book a hotel room in a major city, how to post an ad on CityX, EROS, etc. America is the safest country in the world, and she is unlikely to bump into her uncle or brother in law.
American women will never ever ever starve. The US government guarantees everyone will get a pay check, healthcare, food. This is a disincentive to resort to hooking as a way to make ends meet. That said, do some American ladies escort? Yes! For many of the same reasons as the foreign women, just far less of them.
I am amazed how many hotel rooms are booked in CC, Plymouth, KOP with $400/hr women answering texts in bad English. They cash in and travel home to their villages with a suitcase full of $$$$$.