Hardware Failure of Display

This is not common but does occur rarely with mass produced hardware like these laptop screens. If you have AppleCare, it should cover the repair. If not, it has to be paid for out of pocket.

There is no "valid reason" (how could there be?) for such a failure, they simply happen with a small percentage of the devices. As with automobiles or other complex machinery or electronics.

The reason that the technician cannot provide the exact root cause is that there are numerous integrated circuits involved for the screen to function properly, as well as cabling and other connectors. To track down the exact root cause of the failure would require extensive troubleshooting, perhaps to the device level on integrated circuit boards, which would take considerable time, resources, and equipment. Most electronics repairs for things like this are done by simply replacing the entire assembly. It would not be particularly helpful anyway to find out that the failure was caused, say, by a transistor that failed, replacing a single small part like that would be invasive and risky and might cause other failures.

The detailed technical reason for this failure is very specific.

It is called "Bad Luck".