I don't know where you're from, but you're grossly underestimating how large wilderness areas are.
A few examples:
New York City is 305 square miles with a population of roughly 8,500,000 people.
The U.S. Has 154 national forests covering over 294,361 square miles, and the majority of those square miles have NOT had people "tromping through" them.
In just one state (Idaho) and only one of the wilderness areas in that state, there is 3,700 square miles (2.367 million acres). Or more than 10 times the square miles of NYC.
An interesting story illustrating how long things can go undisturbed in the wilderness areas.
http://www.gunsandammo.com/news/132-year-old-winchester-model-1873-found-great-basin-national-park/
Cliffs:
A rifle was found leaning up against a juniper tree in a National Park, the rifle is 132 years old.. It's estimated that the rifle has been leaning undisturbed against the juniper tree for at least a century... In a National Park.
I'm not arguing one way or another about the existence of Bigfoot.. I'm just telling you if an animal who lives in the forest, wants to remain hidden. It would be easy to do.