With Haskell you just get 5 good ones. You probably don't start with Haskell as your first language but rather move into it after you are a senior in another language. If you are lousy in Java, you probably won't go and learn Haskell or some other niche language.

I would assume that:

(1) Those using niche stuff are less likely to be hiring under the impression that the main measure of skill is years of experience with a language, and

(2) those using niche stuff are, on average, doing more interesting work that attracts more intellectually curious candidates.

As a result, the mainstream firms get worse candidates, and try to compensate by asking for even more years of experience, and asking for years of experience not just with language but specific libraries and other tools, hoping that will get them more skilled candidates, at least for their specific toolchains. But doubling down on that just gets them candidates that are less capable (because even to the extent years of experience are useful, there are diminishing returns, and people who have spent a huge amount of time with the same stack also are likely to be in the “1 year of experience, repeated N times” category, rather than N years of learning and compounding knowledge. (Also, because at a certain point you start making impossible demands, increasing the degree to which the hiring process filters for dishonesty.)

Of course, there is just more of them in the first place. The other effects that you describe might also be true but keep in mind what the base rates are.