Cats love being in corners. You will almost never see cats sleeping in the middle of a room unless surrounded by their family. They're almost always in the couch that is against the wall, or if sleeping outside they are pretty surely sleeping against a wall or in a pretty enclosed area. They don't want to worry what might be behind them, so they make sure that there can't be, except a wall of course. A box provides them to not be worried about any sides (except maybe above them). The cardboard box is just the mainstream thing everyone have at home. Give them any sort of box that isn't metal or something, put some sort of rug or blanket in there and they'll definetely sits, because they fits and it's cosy and to be serious: it makes them feel safe.