If you dig in to those sightings, i think you'll find none are for any real length of time and none are much more but a fleeting glimpse and/or obstructed.
I don't think these things go through the length of time they have 'not existing' if posing for pictures and by that i obviously don't mean literally, but just in general, you don't get too many reports of them that give the opportunity for good pics. They do occur but they're not common, and appear only when they've made a pretty bad mistake which has led to exposure.
Camoflauge > Retreat appear to be the go to for these things and one thing that i think that doesn't get anywhere near enough emphasised when we talk about researchers taking pics, unless the researcher who has the sighting has seen one before, the shock that would come of what would be conformation would outweigh the 'quick let me grab a camera' mentality all day, every day.
Add in the mixer the thinking of potential danger and uncertainty of the situation and i think it's much more understandable why there are so few, if any, decent pics of these things, PGF aside.