Miami neighborhoods

I am in Miami Beach in Miami, and I really do not understand how things get done here. There are not enough restaurants, supermarkets, and groceries to feed all the people who are here. Of course this is false, because they all eat. But you have to go for miles before you will find a normal store for food and stuff. My only conclusion therefore is that there are simply not all that many people living here. The people who spend time here do not live here and do things like stock up on groceries. They come for weekends or holidays and stay in hotels or their apartments and eat in hotels and the like. It is very weird. There is a whole economy that must live of people who come around every month for three days. It is pretty odd that whole neighborhoods can do that.