Because synchronous IO will block the OS thread. If you're multiplexing a bunch of green threads on that OS thread, they'll all stall. That's why Go, for examp

Because synchronous IO will block the OS thread. If you're multiplexing a bunch of green threads on that OS thread, they'll all stall. That's why Go, for example, does use async IO. But it makes the use of it look like sync IO for simplicity. In the runtime, however, it is doing all the async event management so that you don't have to. From a programmer POV this is a good thing, but it comes at the price of a runtime that has to exist. Something that Rust eschews.