For a military, "identifying" a craft is less about saying what something is, and more about saying who something is. If you know that something is a jet, but you can't recognize the make of it well-enough to know whose embassy to complain to for it being in your airspace, then it's still "unidentified."
It's sort of like a criminal investigation. If you know that "a tall, Caucasian male" is responsible for the crime, that's not worth issuing a formal finding about, because you still haven't identified a suspect to pursue. You have some details, but not enough to "go on."
(Also, many things that aren't jets try to pretend to be jets in various ways, many jets try to pretend to not be jets in various ways, and many jets try to pretend to be other jets in various ways. If you just see "a jet", you can't be sure which of these cases is in play. Identification involves finding a non-repudiable signature—a tell that indicates a particular make of device, that cannot be faked by any other currently-known device. Sort of like a (well-made) virus signature.)