Because even companies that did have monopolies on the most essential parts of the economy couldn't do everything and capture everything. A monopoly on intelligence is still constrained by the physical world in which we exist, if it's run by people then they are constrained by their own nature and by the other people that still hold other forms of real power. Society might not be able to function without them but that doesn't automatically mean they are in charge, they exist within that society so if they destroy it then they destroy themselves not to mention the intense opposition they would face if they attempted to do so.