First of all, there's a complication here in that some Indian reservations were created by the Indian nations themselves (particularly in the Pacific Northwest), while others were created as part of the genocide of Indians carried out by Europeans in the Americas.

So let's focus first on the second category. Imagine that some people from elsewhere showed up, rounded up you and all your relatives and everybody else living within a 400 mile radius, killed 80% of them, then marched the rest to a patch of land you'd never seen before and said "you'll be living here now, it's yours in perpetuity". Then a hundred or more years go by, and the descendants of those people say to your descendants "Well, actually, that land we said was yours isn't really yours, and so the problems with resources, jobs, social order, purpose and meaning you folks have had for a while are going to get worse".

And then someone shows up and says "why should these particular ethnic groups get treated differently from any other?"

If you then turn your attention to the self-created reservations, the story is a bit different. Now imagine that some newcomers show up in your part of the world. They seem ready to fight, and you've heard stories from people who have travelled east over the mountains that they are brutal nasty SOBs. The newcomers say that they are ready to strike a deal, so you decide that this makes more sense. You tell them that they are welcome to come and live in this area, but that you want a treaty that recognizes that you will reserve areas for your own people, along with certain distributed rights like fishing. The newcomers sign the deal, apparently quite happy with it. 200 years later, your descendants realize that the treaty rights are no longer recognized, and that the newcomers have changed everything so much with their logging, mining and dams that even the original terms of the treaty don't much sense any more.

And then someone shows up and wonders why your people should treated any differently from anyone else.