I think if there were no boxes in the world my cats would be depressed.
I do a lot of shopping through Amazon and half the time I don't get the contents out before one of the cats is in the box. The baby (who is full grown) has to get in a box if she sees it, even if it's way to small for her. She'll stand in it, sit down as much as the box will let her, and go to sleep.
I can't imagine buying boxes for them to play in when Amazon delivers new ones just about every day. I will sometimes put smaller boxes inside bigger boxes, on their side, to give them more interest, but really all they want is a box to get in.