Missiles are great in their place (and we should have and use more of them), but for close air support there is nothing like a human pilot on the scene.

Missiles can have long lead times beween when the decision to fire is made and when the warhead hits the target (or misses completely). Aircraft may have longer lead times to get on scene, but when they do they can react quickly. That means an aircraft has a better shot at a moving or fleeting target than a missile.

An aircraft can make a quick return pass if the weapon misses or malfunctions.

An aircraft can stay on station for a long time - sometimes hours.

Finally, there's nothing like fighters overhead to keep away the enemy's aircraft. That's an advantage that makes the rest of the USAF's money and issues sorta worthwhile.