I hear ya! Not only do they not give thumbs, heck, they don't even respond. It's like, okay did they even read this? I always try to give green to people who follow me and people I follow, after all, isn't that why we follow each other, because we like each others answers?
They don't bother, aren't aware of the thumb system, don't think it's important, or comment instead. The lowest answers on the thread, or the late ones usually don't get any. Some questions have zero thumbs, and it happens on serious questions and answers. Someone once commented he never answered unless he saw previous thumbs on the question, and he tried to answer first or second. While thumbs are great it can get weird. People just forget, especially if they get caught up in comments. A perfect example was a recent question with a group of 7-8 friends, all agreeing. Each person should have received 7 thumbs,( you would think) that wasn't the case.