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The Germans, Russians, Japanese and British were also turning out really nice airplanes by 1944. The U.S. had one huge advantage.

We could train our pilots completely away from the war. We were also out producing the others.

The British (and some others) could also train pilots completely away from the war — following the “British Commonwealth Air Training Plan”. Most of this training occurred in Canada.

We could train our pilots completely away from the war. We were also out producing the others.

Additionally, we rotated the best pilots back to the State to train the new ones.

Joe Foss, iirc, the Marine Corps' top WWII ace heavily protested being rotated back to the States to train new Naval Aviators (Marines) after achieving 26 kills. His CO told him that he could "come back after you send me another 100 Joe Fosses.

The result was that the top US fighter pilots ended the war with kills in the high 30s (US Ace of Aces, Richard Bong, ended with 40 kills), while their Luftwaffe counterparts (who had stayed on the front line fighting and racking up kills for the glory of the Reich) ended up in the hundreds*.

(*note that countries invariably counted "kills" differently. A full list of aces can be found here)