Except doing so introduces a bunch of needless programming difficulties, including but not limited to the ones discussed in this article. For example, if you make the state machines and epoll loop explicit, you don't have the function coloring problem -- all your I/O requests from state machines to the poll loop (which may or may not cross thread boundaries) are explicit synchronization points with your global business logic, which gives you fine-grained control over how your state machines handle things like request timeouts, resource quota limits, cancellations, execution suspend/resume, and so on. As another example, you're not limited to factoring your state transitions into closures. As a third example, your business logic would have global, explicit control over I/O scheduling, which greatly simplifies end-to-end QoS and request prioritization.