Perhaps it serves an important protective or regulatory purpose which we haven't yet identified, because we still know so little about sleep and consciousness.
An analogy: There are lots of systems in your body that try to prevent and deter you from casually walking on a damaged leg, and we can override some of them with various external drugs, but that doesn't necessarily mean it's a good idea.
Maybe with chronic deprivation + caffeine habit, you're weakening your cognitive function - repeatedly using a misfiring neuron. At least, with no caffeine, you'd feel your memory is hazy and hard to recall, go to sleep.