Whose bright idea is this? I can understand if a modem comes with default passwords specific to that individual modem, but for all modems of the same model to have the same passwords...
It's for manufacturing simplicity since it would be difficult to match a given manual to a given modem or it would cost more and add a step to put a correlated sticker on the box.
Also gives Tech Support a fighting chance to fix things.
Caveat emptor.