Sometimes a member will take the time and expend energy to post the answer to a question asked and that is the end of the thread. No acknowledgement from the OP that the question was answered, no thanks, no solved annotation, nothing. I assume that the OP got what they wanted and did not feel the need to respond.
Is that a cultural thing? Or just bad manners?
narcissism, gaming mentality.
This.
Both at the same time.
It could also be that the helpers on the forum drift off-topic and don't constructively answer the OP's question or become condescending and insulting to the point where the OP just abandons the post.
It seems also to be a misunderstanding of the way the forum operates. Some questions are posted by folks that apparently believe that this is a service paid for by the manufacturer and that they are entitled to help. Some indeed get very shirty when that assistance is not forthcoming in the timescale and exact form that they desire.
The fact that they're requesting attention from a group of volunteers seems to have slipped past many supplicants.
This does happen, but the premise was:
There is an old saying from India, "the hand that is giving, is the upper hand".
Oooh, also... has anyone noticed the trend towards a complete inability to communicate normally? Things like, "read something completely and reply to it", "imagine what the other person might want to know and explain that". "not understand a term or concept and so research it instead of giving deer in the headlights responses"...
But on topic, sometimes I just imagine the OP has turned to the student sitting next to them, got the answer, and nothing else matters. So log out, goodbye forum! I agree, I think it's sociopathic behaviour really.
I think this is just bad manners, you say thank you in a shop when the shop keeper does something so trivial as hand you your change.
I think people sometimes are not aware of the fact that there are real people on the other end.
At the moment I have two threads I spent over an hour each working on that I would have expected a reply on. These seem to coincide with having to have information about what they are doing dragged out of them.
Another trend perhaps from social media is to like every reply made, useful or not, with this being considered polite.
It is a strange new world out there and the older I get the more alien it seems. And don't get me started on perfume advertisements, the rule seems to be if you can't understand it, then it is a perfume advertisement.
Agreed.
IDK could they really be that dumb? This isn't the only support forum in the world...
I'm sure that sometimes it is related to school work.
The OP is generally cagey and does not want to be seen acknowledging the contribution that others have made to the project and wants to quit the scene as soon as possible.
Oh, not "dumb" but environmentally driven to communicate differently; my analogy is that old farts like myself grew up analog. But the new generation is growing up in "packets": they think in small segments, communicate in bursts, and generally seem to be confused by an older generation.
Sadly, many of us older folks created this enabling "packet" communication monster.
Very often, I suspect the OP never comes back to see the responses.
On other forums I used to cultivate a habit of never responding to a poster who was a first timer. I often spent a lot of time researching their problem, formulating a correct answer that I tried to explain at a level suitable to their knowledge, but it would never be read (poster's account would still show just the one visit).
So to keep my sanity I started only doing that for people who seemed serious about getting their problem solved.
These days I take a wider view: maybe even if the OP doesn't see my answer, it can help others. Is this useful? No idea, but it makes me feel I'm not wasting my time 
But yeah, it would be nice to know if you helped someone. Also, some people will thank you by sending a private message.
yes and yes
As they say:
One's lack of intelligence may be judged by one's actions .
So excited the gadget works they forgot to return to offer thanks.
(At some point in your younger life even you did that.)
Possibly, definitely.
You folks are "preaching to the choir". If they couldn't stick around to say thank you, what are the odds they're now reading this thread and self-identifying? Not likely, I'm afraid.
They will go so far as to delete a thread, or at least their own parts, as soon as they receive help.