Put a plastic bag or cardboard box on the ground and a cat will gravitate towards it like metal fillings towards a magnet. The big question is why do cats consistently like plastic and paper items?
One theory is that cats are curious, but that doesn’t explain why cats don’t jump into bathtubs willingly. Another theory says that cats like the smell left behind on the plastic or paper, or that they enjoy the crinkling sound these materials make.
Whatever the reason, cats will always find a way to climb inside boxes and bags just for the fun of it so you might as well save money on cat toys and just pick up a free cardboard box instead.