USB differential signal from single pin?
USB is not always differential. There are signalling states that require both D+ and D- to be low.

Additionally D+/D- are used for other purposes, such as performing reset and speed detection which require sense resistor configurations to be changed.

The pin isn't "wasted", it's "required". I suppose you should think of it less like a differential pair, and more like a clever two-wire communication protocol that is sometimes used differential to improve speed and immunity, and sometimes single ended to do control stuff.