Europe was never strictly occupied by this one group of people.

Proto Indo Europeans are a group of people. Sure they warred with eachother but as distinct European nations emerged they frequently banded together to keep out invaders. The Moors, Ottomans and Mongols were repelled from Europe thus preserving Europe for Europeans.

Native American tribes were not so unified. The Iroquois alliance nations were the closest chance to a united front and they were still too far behind technologically, population size, and immune system wise.

This argument is a little silly though because the spanish had already started to spread disease and depopulate the tribes of north america before the English showed up. It truly was very un populated in large parts of North america When the English arrived. Your case is stronger if you're talking about pre Spanish contact.