You're right. They are working on large volumes mostly moved in and out of the country (however they manage their exchanges), as opposed to people going to them to buy or sell currency in cash (in relatively small amounts), which is why their volume is greater. And of course, when people are trying to get cash out of the country, they are paying more and usually dealing in cash and not bank transfers.
Most expats who have been here for awhile should have the same kind of contacts - Florida street, "jewelry stores", t-shirt shops and other likely places should be a convenience, from time to time kind of place. The "real" currency exchange places are much more private and secure, and you can easily get money out of your home country to here if you make such a contact, paid to you in your choice of dollars (probably euros as well, but I've never asked) or pesos at a great exchange rate. Of course it costs you 1% or 2% depending on your transfer which has to be figured in to the real exchange rate you get, but it's still better than Xoom.
The place where I go would certainly buy or sell dollars for me in cash, but that's because I'm a client. There have been times where I've needed to buy a few extra pesos with some cash I had on hand when going to pick up my wire transfer, but I've never gone to the place just to change a few hundred or even a thousand dollars. They certainly don't want a lot of "petty" traffic selling a few hundred dollars here or there.
Florida is just an easy access point to the blue market for tourists and locals who want to take a few hundred dollars out from under the mattress.