Not sure about NL, but in Switzerland you can buy those codes with an SMS. I guess many people want to do it quickly, and they might have already written the address of the recipient. So just getting a code it much faster, you don't have to enter the address somewhere.
I think the main issue is that regular post has addrrss info of the recipient. So the whole logistics chain is geared towards that. If you remove the need for address info for a (small) subset, you'd have to have two logistics chains somewhat intertwined with each other...
