It gives a constant stream of novelty and instant gratification, two things the human mind is deeply biased to crave, delivered through an interface produced by decades of A/B testing and applied behavioral psychology.
It tastes good. People really like it. Given the choice between rice cakes and chocolate, you'd probably choose chocolate. Chocolate manufacturers test chocolates and release the more successful ones.
Does chocolate have teams of behavioral psychologists designing every aspect of the experience of its acquisition and consumption in order to create a situation in which a huge chunk of our society is unable to stop compulsively consuming it to a point where it's negatively affecting their lives?
It absolutely is an addiction machine.