The difficult Art of understanding customer requirements

When it comes to quality of supply, the first expression you hear is "customer requirements".
The customer's requirements have been the Achilles' heel in the process of obtaining parts by metallurgical processes for many years, but why ??
Why the main characteristic and needs for supply is often unknown even by the customer himself, or as some say, not even the customer knows what he wants.
I believe that is question that moves an entire sector, but after all this is a problem or a defect?