They do, but only at the expense of interoperability. Any "green thread" solution breaks down once you have to invoke code written in something else while allowing it to call you back. Async futures, on the other hand, can map to any C ABI with callbacks.

So until there's some standardized form of seamless coroutines on OS level, that is sufficiently portable to be in wide use, we won't see widespread adoption of them outside of autarkic languages and ecosystems like Erlang or Go.