Why don't executions happen by massive mechanical impulse, or at least have the option to do so?

Most modern execution methods like electric chair or lethal injection rely on specific design than can easily fail (such as chair design, drug malhandling, or a botched injection itself) and that take a period of time to complete which can cause suffering. Why not give the option for a mechanical alternative, after a sedative, that would simply deliver a sufficiently high moment or impulse to overwhelmingly destroy the head/brain and cease consciousness, with far lower risk of pain or failure?