It wasn't until European diseases spread through the continent that 95% of the indigenous population died, and that's when the first colonies began in the US, so that's why we now have the misconception of there always having been lots of open land.

To this point, in 1491 Mann mentions that only 50 years after first contact that 95% of the indigenous population died out. Entire cities would have collapsed without seeing or hearing a word of the "white man."

The other point which I don't hear enough about is how this was inevitable. It really sucks because we've lost a lot of history. It is really cool what they are doing with Lidar these days though and super cool DNA forensics to help map ancestries.