Fragile human pilots are no longer needed. During combat, the loss of a military aircraft is a technological and fiscal blow, but the loss of a pilot is far worse--personally it's a loss, politically it's terrible, and financially it's a disaster, since pilots cost millions of dollars to train and keep flight-ready. Human pilots also require life support systems in aircraft, which increases their weight, and limits their flight envelope--without the pilot aboard, an X-47B-type aircraft could theoretically perform all sorts of crazy aerobatic maneuvers that a human pilot couldn't withstand.
The title confused me. The XB-47 first flew over 50 years ago: ...and it had a whole crew. Unmanned planes have their advantages, but it's usually the airframe, not the pilot, that limits their manueverability.
