The Bad Guys shoot back
"An A-10/Harrier can get in close on a strafing run within a couple hundred feet, "
The Bad Guys, even irregular guerilla forces usually have heavy machine guns that can shoot back if a ground attack aircraft gets within "a couple hundred feet". Pilots that want to go home after work shoot up the Bad Guys from a few kilometres away using Hellfire and Brimstone missiles from well out of range of heavy MGs and shoulder-fired missiles. As a bonus they don't accidentally walk cannonfire through friendly forces when the nose of the aircraft drops in a rough air pocket.
A sniper wouldn't charge his target to bayonet them, why would it be necessary for ground attack aircraft to knife-fight when they can stand off beyond visual range and turn the opposition into mince without allowing them a glimpse of the weapons platform that is destroying them?
As for the Harrier, a triumph of 1970s engineering, it is slow and short-ranged with a limited payload compared to regular catapult-launched aircraft such as the F/A-18. Its big advantage for the USMC is its ability to operate from rough airfields and short decks on the Tarawa-class and new America-class Marine assault ships, America's other aircraft carriers. The F35-B can also fly off those short decks as well as being faster, longer-ranged, smarter, stealthier and generally isn't falling apart due to the age of the airframe hence the USMC's keen interest in getting more of them as soon as possible to replace their forty-year-old Harriers.