Sentry guns on aircraft

Why don't military aircraft have sentry guns (autonomous sensor-guided guns) to fire at incoming missiles and attacking aircraft? Applying such weapons to aircraft (as opposed to ground vehicles) would be fairly low risk of friendly fire as they would only be turned on when the aircraft was operating independently. It would be possible to add several sentry guns to an aircraft to cover all angles. One might imagine that this would be particularly useful for UAVs, which may not be able to replicate human dogfighting abilities for some time. The technology certainly exists.