This conversation is very much like saying, "I don't believe in gorillas, cause I haven't seen one in the wild."
Animal X's existence in a region is not dependent on anyone's belief-- this isn't religious faith, this is animal biology and habitat. They either exist, or they don't. "Belief" is not relevant. This isn't a supernatural question.
As far as animals creeping along with ninja like stealth, there are scads of them in the Sierras. Some of them are called mountain lions. Sneaking is not that impossible. Heavily camoflauged hunters do it all the time, just for fun. Preditors and other sneaky animals do it for a living.
The terrain is so intimidating at high altitude, that hikers stick pretty closely to any trails. It wouldn't be too hard at all for a competent biped animal to hang out up there, just a few hundred yards off major hiking trails, and never be seen due to nocturnal habits. A lot of that terrain is so unyielding and rocky, that a halfway smart creature could easily choose to leave no traces. The mulies in the national parks are practically tame, and I easily approached to within 30 yards of groups of them in the waist high brush. They would be the food of choice for any omnivore or carnivore up there.