We don’t observe carbon dioxide being unburned for the same reason that we don’t observe heat flowing spontaneously from the cold to the warm room. Neither process would contravene the first law of thermodynamics—no energy would be destroyed or created—but they would reduce the entropy of the universe, which is forbidden by the second law. All such reactions that result in an increase in entropy are called spontaneous. That means they will proceed as long they have received the activation energy required to start them.
