Because: A) poor nations all over the world receive funding from the United States in order to help secure our interests and stabilize regions of concern to us, that’s particularly significant when those regions include areas entirely surrounded by a state. B) They aren’t separate to a degree that matters for economic development. Many reservations are bound to some US or even state laws in exchange for certain things. They can’t decide to radically change their governance the way like Mexico can especially when it comes to things like building a modern standing army. They can’t create a hard border with the US. And C) their land sucks because whenever it turns out we accidentally built a reservation on land which has economically valuable resources we move the reservation to land that doesn’t or we starve and buy them out of it or something similar. We have created a system of hopelessness and routinely undermined their ability to create a society that can economically sustain itself without us.