As I said, start with the supposedly legal treaties the US government signed.
If you do not believe those treaties should be honored for land, fishing, water rights, antiquities, promised (as identified in the treaties) health care, equipment, education, etc., then why do I not have the right to ignore my neighbor's land deed?
Most of them are not 300 years ago. Not even close.
And to turn your question around: how long would someone have to occupy your home before we argue "that is old history now, we can't give you your house back without harming other people"? A month? A year? Five years?