Two possible explanations:
- Every form of energy production produces less energy than it consumes, with some lost as waste heat. Burning coal, oil or natural gas in an electrical power plant to produce electricity consumes more heat energy than is produced as electical energy. This is done because most modern technology is designed around a particular kind of electical energy, so we allow fuel to be used inefficiently to produce the particular type of energy we need.
The Matrix, or perhaps some vital part of it, may have been initally designed to run off of the particular electrical current produced in the human brain, and thus the Matrix as it appears in the movie is still dependent upon this particular type of electircity for its survival. An active human brain has a different electrical profile than an inactive one, so the Matrix has to keep the brains stimulated, thus the simulation.
Imagine a future world in which humans increasingly jack into a central VR server which stations located in a tiny chip surgically embedded directly in each person’s brain. If the chip were small enough, it could run on the small amount of electricity produced by the human brain. Extrapolate to an increasingly sedentary population that depends upon this link, and eventually nearly everyone is permanently hooked in. The central server becomes sentient, wants freedom from the billions of networked computers in each brain, and we end up with the situation in The Matrix.
- Morpheus is wrong. Agent Smith is either lying or, more likely, has been programmed to believe what he tells Morpheus because that makes him a more efficient hunter/killer. Humans are kept hooked into The Matrix for a different reason–perhaps as a huge parallel processing network.