The problem with readiness-based IO for disk is that without starting the request it doesn't know where it needs to read on the disk to fetch, so it can't possibly tell you if the data's ready. If you were to limit file IO to streaming rather than random-access it could potentially work, but that's not the POSIX interface. Since pipes and sockets are streams they don't have this issue.

I'm not defending the suckiness of the AIO situation on Linux, but I understand why it is to some extent.