I think it's kind of ridiculous to say you can't use _ because it is a valid identifier. You could just have the pipe _ shadow the identifier _ if said id is present. I suspect the bigger reason is because of underscore/lodash being in wide use.

Suppose you had "const _ = 42; /* more code / return x | f | g(11, _);". This would be semantically equivalent to "const _ = 42; / more code */ return ((_) => g(11, _))(f(x))". Since it is a new function, shadowing is not forbidden.