That's because it's not a constant machine, and is in fact interrupted constantly, sometimes by an early death. If we actually come up with a machine that is truly constant and stable, it will be the end of civilization as we know it. People might think that they would never fall for it and would never just stay in a room hooked to something like that but that comes from a misunderstanding of the power this hypothetical machine holds. Looking at the lengths a heroin addict will go to secure diminishing returns should give anyone a hint of the terrible potency of such an invention.