Concurrency is hard. If you don't have anything better to do but wait on a single file operation (which, in a CLI tool, you might not), then a synchronous call is just fine. If you do have multiple I/O operations to issue at once, that can still be synchronous with writev().

Most forms of async programming are also unstructured, which is bad for correctness, but also for performance since it can lead to priority inversions.