They lost their highly trained experienced naval air arm at Midway. They then had to move their training cadres into the fleet as replacements. When they were gone, they were stuck with naval aviators who could barely fly, take off & land was about it.
A similar thing happened to the Luftwaffe. In July of 1944 the Germans produced, either newly manufactured, or depot level refurbished, something like three times as many single engine fighters as they deployed on any one day during the Battle of Britain. But looking at the combat effectiveness of the Luftwaffe, you would think that in the Summer of 44 that it had only a small fraction of the fighters it had in the Summer of 40. Because the highly trained pre-war pilots of 1940 were dead, POWs, maimed, or moved up the chain of command.
Faced with overwhelming pressure to get bodies into the new aircraft, and into action, immediately, and facing fuel shortages so crippling that planes were towed to and from the runway with oxen, to save the fuel consumed by taxiing, the Luftwaffe cut flight training to almost nothing. These scarcely trained pilots tended to have VERY short operational careers, and the Luftwaffe was never able to break out of the vicious cycle.