I do try to rate back the people that rated mine and also give feedback. I'd say an eye-catching thumbnail does the work of getting people into clicking but ultimately it's up to them if they want to rate or not. Some people abstain from voting games if the rating is going to be not-so good. It also could be that not that much people are actually playing the game enough to feel that they can give constructive criticism. Imo people play for a couple of minutes and in my experience they don't really read descriptions, so the game better be engaging in the first seconds or they leave.
(On another note, is karma not entirely related to ratings? I've rated & commented on 34 myself and i'm far from the top)