That doesn't really work either. Human brains are not great at computing unless you are looking for "good enough," results, and only on some pretty narrow fields, facial/speech recognition, some physics interactions, etc. But worse than that... we're kind of using them. If they wanted us to compute, the whole function of the Matrix is just taking up run cycles. And you can't just coopt them during sleep, we need the rest periods ,or we literally die. Only one answer makes sense to me, it's a nature preserve. They didn't want to be responsible for destroying their creators, and the only other sapient species known to exist. So they build the Matrix to keep us docile. Then, the energy reclamation actually makes some sense. They're never going to be net positive, but assuming they are having difficulty keeping their society powered, they would be incentivesed to reclaim every watt of power they could from us to reduce our burden on their grid.