This doesn't work well in practice, because part of structured concurrency (at least as Python Trio introduced it) is to have a sane story about cancellation and timeouts. There isn't a safe way to cancel an OS thread or process short of some prearranged handshaking-- the thread or process being wrapped certainly needs to cooperate.

Coroutines give you an easy way to manage cancellation. Essentially, any yield back to the scheduler is an opportunity to inject a cancel into the task. The code run within the task doesn't need to do anything special (beyond not starving the scheduler).