Though I can't speak for this perpetrator, I think people have a psychological inclination to grab what you can, sort of a vestigial instinct from the vast majority of human history in which, for most people, resources were scarce. It's a form of hoarding behavior. And, of course, it's probably much more common among those who grew up hungry and learned to treasure food. I've been at buffets and had to remind myself that it, whatever "it" is, will almost certainly still be there if or when I come back and that, if it isn't, I'll make do.
Then again, there have been numerous recorded incidents of people fighting over crab legs at buffets, so I can almost understand the hoarding tendency. But finger sandwiches?