I suppose for the same reason that every time they find an old skeleton it's not an Indian but a white guy that got here first. (Kennewick Man)
My impression is that 99.9% of the time when they find an old skeleton in America, they think it's a Native American Indian.
If it's older than 6,000 years old, that's usually a correct statement. Luzia, a 10,500 year old skeleton found in Brazil is an African. It appears that today's American Indian/Native Americans were the late-comers to the Americas.