system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:In the Pacific Theatre in WWII, why did Japan run out of experienced pilots but the U.S. didn't?
gpt
It is often for the same reason that Axis aces had much higher kill counts than their Allied equivalents. Japan had spent much of the lead up to the war investing heavily in pilot training programs for their carrier air forces, and the war in China gave them combat experience. When the war began, the pilots of both the Imperial Japanese Navy and Army were highly skilled and performed well against Allied pilots. However, once the war started, and these highly trained and experienced pilots and aircrews were slowly whittled away. Often, the Japanese would keep their best pilots on the front lines until they were killed, while the United States would often pull back pilots to train future pilots. 

Furthermore, once the war started, and resource limitations became prevalent, and the Japanese began taking losses, the Japanese couldn't give their pilots the same amount of training that their pre-war training programs gave. Not enough aviation fuel to fly training missions, etc. Moreover, the US simply had the resources to dramatically expand its pilot training programs, when Japan was basically already at its limits when the war began. 

Effectively, this meant that while the pre-war core of Japanese pilots was some of the best in the world, they had little ability to replace losses from this core of pilots. They couldn't spare pulling back pilots from the front lines to train new pilots, and this made their core of pilots was simply whittled away. Eventually, once this pre-war core was exhausted, there was very little left.