The runtime being a library instead of a language/compiler level feature. Custom runtimes is necessary for systems languages as they can have specialized constraints.

EDIT: Note that it's the presence of a supported runtime for the async operation (e.g. it relies on runtime-specific state like non-blocking IO, timers, priorities, etc.), not only the presence of any runtime.

> What prevents the runtime from preempting a task?

Memory efficient runtimes use stackless coroutines (think state machines) instead of stackful (think green threads / fibers). The latter comes with inefficiencies like trying to guess stack sizes and growing them on demand (either fixing pointers to them elsewhere or implementing a GC) so it's not always desirable.

To preempt the OS thread of a stackful coroutine (i.e. to catch synchronously blocking on something) you need to have a way to save its stack/registers in addition to its normal state machine context which is the worst of both worlds: double the state + the pointer stability issues from before.

This is why most stackful coroutine runtimes are cooperatively scheduled instead, requiring blocking opportunities to be annotated so the runtime can workaround that to still make progress.