There are many other animals that exist today that were once thought to be mythological monsters. The mountain Gorilla, for example had long been known to the inhabitants of Rwanda as a huge dark man like beast that swung through the tree tops emitting a fearful cry; scientist tagged him for the first time in 1901.

Other examples that were once dismissed as mere folklore include the Pigmy Hippo, the Snow Leopard and the Giant Panda (China in 1936). All these animals from the far corners of the world existed only as a myth until they were discovered by explorers.

The ornithorinx, a strange mix of duck and beever found in Tasmania, the Saola, a mix of goat and antilop found in the remote forests of North Vietnam, shows Mother  Nature has more imagination than we would have imagined ourselves.

The areas where Bigfoot is supposed to live are mostly highly inaccessible or covered with a dense vegetation that prevents any spotting from the sky.
The Pacific Northwest where Bigfoot is supposed to live, hides 73 aircraft lost from Northern California to Alaska since World War 2  (FAA figure). 

The other areas: the Himalayas mountains, the Borneo jungle, the Pamir-Altaï, the Amazon Forest are equally inaccessible to modern means of investigations and hostile to man.

Small groups of very cautious animals could easily remain hidden, even close to humans dwellings. All the once hidden or lost species are primitive animals whereas Bigfoot is a more evolved creature that can hide much easier from man.