What is clear is that if you open threads (if they don't suck) it's easier to get merits. I don't know if they know it and that's why.
No, I have seen alot of good thread that didnt get a single merit but it depend on the quality some merit source look out for before giving merit.
"Thank you" after they got posts with answer to their question or a step-by-step guide with solution to problem they had. Or for example they claim that "the earth is flat" and want to discuss that, but never respond on opposite facts other users present.
I cant explain why they behave like that. Sort of a posting to get more activity on account.
You are right but this don't happen in every occasion and it think make some newbie not to say thank you is individual understanding or the excitement/comfort of finding solution to the problem which seem to be much of a burden.
I have seen a newbie that had issue with MetaMask sign message which I help out and she said thank you.