You are correct in that there is no reason for the user to have the key for the preinstalled Windows and this has been the case since Windows XP. Before Windows 8 the mechanism required a code in the BIOS to verify the system was entitled to have Windows installed. That code was not the key itself but was different for each manufacturer. For those systems they did have key label but it was not the same key the manufacturer was issued to build and license the system. Even then there was no value in knowing the build key for OEM systems as it could not be used for anything.
The UEFI BIOS now allows the key itself to be stored and Microsoft no longer provide a second key on the label. It is still the case that there is no need to know the key because Windows 8 setup can read the key directly if it exists.
I still build my own computers so I can have the exact spec I want but I no longer build computers to sell. It was difficult to make any profit and I was tied to provide support, far easier to guide a user to buy a PC on line and the user has an alternative support option.