OK...I know you say they're supposedly highly intelligent...but
With the gazillion or so (a made up stat) game spotting cameras set up by hunters every year, out there in BF's presumed habitat, wouldn't ya think that one of them would accidentally catch a frame with at least part of a BF as evidence?
Also, why are all supposed pics of BFs, UFOs, Alien beings, spooks, leprechauns, chupacabra, etc,etc,etc...blurry as hell...