QKD does defend against active man-in-the-middle. Working from the premise that qubits in general cannot be cloned (the no-cloning theorem), a protocol can be devised that can detect tempering by an adversary with unlimited computational power and technological abilities. BB84[1] is such a protocol, which is also used in commercial QKD products.
Of course it doesn’t defend against attacks on the devices, buildings or people; these are likely higher ranked risks than crypto attacks for many organisations.