Anti missile defense other than jamming and decoys do not exist for airborne platforms.
US drones operate in an uncontested environment let them try fly over Russian or Chinese airspace, the soviet shot down a bloody U2 thy can hit a globalhawk or a reaper.
HARM is typically used to engage a SAM site's radar, not the SAMs themselves. Intercepting a missile is a bit harder than nailing a stationary emitter. Plus, many SAMs use IR sensors, so if you don't detect the launch signature, it's hard to take defensive actions. DIRCM, laser based dazzlers etc can help, but not if you don't detect the launch.
US drones operate in an uncontested environment let them try fly over Russian or Chinese airspace, the soviet shot down a bloody U2 thy can hit a globalhawk or a reaper.