>>510024393 (OP)It's to preserve the legal basis of the treaties. If the reservations were removed, that would mean that the tribes are not in fact sovereign entities able to make treaties with the United States on coequal terms. That would reframe all the treaties as illegal contracts with people who were under American jurisdiction and thus void. All the land annexations going back centuries would be legally void and have to e reversed. The reservations and the semi-sovereign status the tribes hold are legal fig leaves in that regard.
Also cultural inertia. US-American Indian relations progressed to the reservation system before public opinion changed and people thought that maybe we should stop treating these people so horribly. If that public opinion change hadn't happened, the reservations probably would have been abolished, sold as private property, and the tribes converted into legal trust organizations that do some federal aid coordinating.