There could actually be several reasons for this.
First, I'll begin by saying that while it's nice, a "Thank you" is not required nor expected from anyone in the platform. Most of the people who answer questions, myself for example, don't really expect a "Thank you" from the person who asked the question I answered. The same goes with those who ask questions. Sure, it's nice to get them every once in a while, but I don't take it against the other person if they don't want to thank me for replying to them. It's a decentralized blockchain, after all. So, just because you answered a question doesn't mean that the other person has to thank you.
Second, it's possible that the person thinks an upvote will suffice as "Thank you." Personally, if I don't reply with a "Thank you" to someone, it's because of this. Why? It's just that the way I see the platform, comments and replies are for when you have something to say other than a "Thank you." If you just want to show your appreciation, the best way to do that is by upvoting. So, it's possible that a user has already given an upvote and doesn't think that a "Thank you" is required further.
And finally, we have to remember that commenting costs Resource Credits. There are accounts who have to be really conservative about the number of comments or replies that they make everyday because of their limited amount of Resource Credits. For example, new accounts only have enough Resource Credits for 19 comments before they run out. So, in their case, it's better to reserve comments to when they have something meaningful and valuable to say, other than "Thank you." Don't get me wrong, I'm not disregarding how important it is to say "Thank you". But, as I mentioned before, at least in the blockchain, an upvote is just as good if not better than a Thank you. They're also cheaper in terms of Resource Credits.
So, there you have it. It's possible that not a lot of people say "Thank you" in Musing.io because they don't see it as necessary and feel that an upvote is a better way to show their appreciation, instead of having to write the words out right. Remember, every action in the blockchain costs Resource Credits, and for some, saying a simple "Thank you" can get very expensive.
Hope this helps.
Same can be said for people who don't reply with "You're Welcome" to thank-you's, mostly because this kind of responses are already implied.
There's also a misconception spreading around here on Steem that short contents (comments/replies included) are considered "spam". So when people don't have anything to add beside the generic "Thank You", some opted not to comment because they are scared to submit only a 2-word comment that might label them as a spammer.
(There was even a bot created months ago by someone whose sole purpose was to downvote 1-word comments/replies. That bot is already retired though. So you see, their fear may be somewhat justified in this case XD)
Because what they are looking for is actually the answers and views that are different from the questions they want.
Actually the questioner also has their own answers, the problem is that sometimes they want to add references from other people with different backgrounds.
And I'm sure, not all questioners are not thankful to the answerer, there are some questioners who thank the answerer for giving different input and point of view about their questions, not infrequently also providing new information for the questioner.
I didn't know that was expected?
I think, in most cases, if an answer received is engaging to the questioner, would catch their eye if you will, They would address it's content.
If it means to you so much, I will comment "Thank you Kanye, very cool!" on your answers.