I agree with you 100% babydust manners are free and in an ideal world there would be no need for this post. However, it could be that they are fed up with getting stupid, unhelpful and useless answers or having their thread hijacked, so they just think 'oh well, it's gone past the point where a thank you would be noticed (perhaps they signed off and didn't sign on again until the following day).
I'm sure we are all aware that there are some questions that are just designed to get a rise out of people, so even though they appear genuine at the time they are just a wind up ~ hence no thanks proffered.