Well my thinking is this. There are and estimated 15,000 - 17,000 black bear in PA. And there have been hunters who have yet to see one. What is the percentage of times out of total times in the woods has the average hunter seen one. And the deer population I believe is estimated about 400,000 but I've hunted about 30 days this year on public land and have only seen about a dozen or so.
Then you have a trappable population of bobcats to which I've only seen 4 in my 45+ years of being in the outdoors. Coyotes - I've only ever seen 2, yet tracks are everywhere. So the idea that cougars are here, to me, does not seem too far fetched. And as for seeing tracks from them, if they are here, they are few and far between and they are very reclusive animals so not seeing tracks is no reason for doubting sightings upon sightings from educated, knowledgeable hunters. Heck, even in states where there is a huntable population, it may take days of driving back roads in vehicles with scent dogs to cut a track.
As for Sasquatch - I've yet to be formally introduced. But I don't think they've migrated from Mich's upper peninsula yet.