That has nothing to do with Haskell's expressivity and everything to do with its full purity. If you look at, say, OCaml, the code (or at least the libraries) must also be aware that it's running in a fiber, just as in Java, Go or Erlang.

While full purity has its virtues, it also has serious drawbacks, and it's unclear at this point whether or not at the end of the day purity is a net-gain, neutral, or even a net-loss.