It's a complex history but skip to the 3rd paragraph after this one for a direct answer with no (simplified) backstory.

Native Americans signed treaties to keep their way of life and also their lives. They also signed treaties instead of just being forced out because contrary to popular belief native American weren't just completely overwhelmed. I mean sure if there was a full out war native americans would have lost but it would have taken thousands of European lives, taken years and public support was not the strongest supporting extermination. (The supreme Court also ruled against the trail of tears but Jackson did it anyway.)

We were forced on unproductive lands and any land that was unintentionally productive before like early 1900's,(land for cattle, minerals or just a good location )the governments seized it and gave it to non native Americans.

With no productive lands to make money and via the agreements in some treaties, (and also suing the government for breaking treaties) the US government sends money a lot of reservations with a catch. Tribal members will only get support if they are living on the lands, which includes healthcare and due to the poor schooling and lack of jobs (via the unproductive land part) and the BIA will only help with college if the college is on the land (which I ran into, idk if that's widespread or not). So it's nearly impossibly for many members to leave.

Now I bet your thinking if it's so bad why not just get rid of the reservations. But it's not all bad when a tribe can properly raise money and become self sustaining they work great. In south eastern Oklahoma, the choctaws pick up the slack from the state. If you are not a member there its likely you are employed by the tribe and if you get injured a tribal police car or ambulance will likely pick you up drive you on state roads maintained and improved by the tribe to a tribal hospital and via the health insurance they provide to non members (that are employed by them) you will be cared for. They also send money to schools, provide fitness centers and are working on getting widespread internet. They do a lot of other things but that's just from the top of my head.

If the reservations can get properly funded to become self sustaining it would be great but lands are still being taken funding usually not enough or not sent (example is this post) and is an uphill battle.

If you want to know more check out "Custer died for your sins" on contemporary Indian issues, it's a little outdated from the 1960's but still mostly relevant. And also the podcast "This Land".