Although I think "impostor syndrome" is vague and over-diagnosed, it's a natural reaction to a diseased society such as ours.

Capitalism tells you you can be anything you want to be, if you're good enough. Of course, most people are never going to get anything good--the decent opportunities were mostly allocated before we were born. Watching this, the naive conclusion is that only the very best are going to get anything, and that everyone else just sucks. (And, if 99.9% of people who try to do something suck at it, it's just reasonable to assume a high probability that you also suck. Priors. Self-perception does not typically have enough resolution to conclude otherwise.) The other possibility is that society is deeply and thoroughly corrupt, but to voice it makes a person sound bitter... and bitterness, at least in the US, is extremely socially unacceptable, especially when one is right.

If it were a college, it would look like this.

    Person: Hi, Labor Market University Dean. I'd like to major in writing.
    Labor Market: No can do. We've got too many writers. 
    Person: I suppose I'll major in... machine learning sounds fun.
    Labor Market: No slots left. All taken.
    Person: What about business leadership?
    Labor Market: Hahaha, no.
    Person: Well, I have to do something. What've ya got?
    Labor Market: It looks like we've got room for you to major in... subordinate bullshit, subordinate bullshit, and... subordinate bullshit.

Of course, to speak of impostor syndrome usually implies that the person actually is competent (not an impostor). And so, it tends to be most prominent in people who (a) are quite good at what they're doing, and (b) have beaten the odds to achieve some recognition and success. The problem, at that point, is that you start to see so many charlatans who get just as much in the way of rewards as you do. What this establishes is that worldly success, rare as it is in anything worth doing, is not indicative of skill or talent, at least not in any field where there is even an iota of subjectivity (athletics are a different story). There's too much corruption and there's too much noise.

As humans, it comes naturally to find patterns when they aren't actually there. If we fail, it's too easy to accept society's narrative that we deserved failure and aren't really fit to be more than a business subordinate. If we succeed, we see both (a) the failure of people as good as, or better than, we are, as well as (b) undeserved success of incompetents even at the highest levels. The first case is one of perceiving a spurious pattern; the second is a case of watching the pattern break down and concluding that external indicators of one's competence carry almost no signal, thus falling back into an I-know-nothing state... which tends to make a person feel incompetent... because how good can you be if you don't even know whether you're good[1]?

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[1] In reality, there is at most a weak correlation between one's competence and precise confidence thereabout; however, if we make certain assumptions of rationality that sometimes hold and sometimes fail, we are led to expect that they would correlate.