Vietnam caught the USAF completely flat footed and the USAF had to buy Navy planes... Phantoms and A7s for ground attack. Even after that embarrassment they still tried to go back to their favorite hobby horse... air superiority and strategic bombers only. When I was in Nam the Navy and Marine support would come in low. The USAF planes were ordered to stay high.
In fairness, it's right in their fight song....
Off we go into the wild blue yonder,
Climbing high into the sun