This issue was addressed in a short story included in the book "Tales from the Mos Eisley Cantina". Davin Felth
, the Stormtrooper who said "Look, Sir, Droids." had been training to be an AT-AT driver and developed a technique that would allow the AT-AT to defend itself from airborne attacks on its vulnerable legs. Instead of accepting the new tactic, the Empire reassigned him as a Stormtrooper. Nobody ever said the Imp brass were smart.