I think this is an interesting question. :)

I would guess that the reason you have seen a lot of NFPs and STJs is because Te is more inclined to value external input on this kind of question.

I'm an INTP, and although occasionally I have been inclined to double check how others might perceive me by making one of these threads, I perceive the possible gain (weighted against the time and effort of covering all the relevant information; if I made one it would be very detailed and would try to clarify exactly why I was confused) as rather low.

My type needs to make sense to me; other people's opinions or judgments are interesting and can be helpful, but ultimately they are less important.

I do - and have - asked for input from people I know when I feel like I have trouble distinguishing between possibilities (I would guess that often it is strong Ne that can muddy the waters), and they put me on the right track, but figuring out which type is the best fit seems like a very internal process to me.

Does that help? :)

I guess I would say that Ne can easily make one think that they need external help because they have trouble narrowing it down; Te in general likes to be able to point to exactly what it is that makes something a good fit (descriptions, test results, external opinions, etc.); Ti that is lower in the stack might make someone wonder if they are applying labels correctly, and Fe in probably all positions likes to hear that the way they think of themselves matches what everyone else thinks.

Also for STJs a major factor is probably Si; when someone is new to MBTI they have very little experiences and references for what is useful or normal and what is not. This is less of a problem for Ni dominants (and from what I hear most INTJs - for example - do not doubt their type much at all), but if Si is strong then it is probably easy to feel disoriented by the information available and not know exactly where one fits in (and yet feel strongly that it is important to know).