Pure functional languages model IO with monads, which are very much functional (function composition within a context) and IMO easy to use. This ease of use is probably why JS adopted the pattern with native promises -- async/await. They fit well into otherwise imperative code and people seem to to like them.
Pure functional languages can very well have normal IO. It's Haskell's lazyness that mostly forced it to use monads for IO, for better or for worse. In a pure FP language with strict evaluation semantics, IO can easily be implemented as a pure function foo -> (World, Input) -> (World, Output).
