Just to add my opinion, which mostly covers things already said.
First point, the US got into the war later than the Germans, Russians, British, Japanese, etc. so they had less time overall in combat. While just about everybody else was well into it in 1939, the US stayed out until nearly 1942 (and took a while to ramp up even after that).
Secondly, we did rotate pilots out unlike most other countries (Germany being a prime example). Add this to the first point and that accounts for a large percentage of the difference right there.
Thirdly, although at the end of the war I’d say the US had the best collection of fighters going, we had some real clunkers at the start of the war. The P40 and Wildcat were tough, but lacking in performance compared to Zeros and Me109’s and such. And don’t even mention the Brewster Buffalo.
Fourth point, a lot of the German aces racked up kills against Soviet pilots that were inexperienced and flying inferior aircraft. And don’t take this as fact, but wasn’t there a tradition of assigning kills to the squadron commander sometimes even when they were made by someone else?
Fifth, by the time the US had better fighters in substantial numbers in Europe, there were lots fewer German planes to shoot at. My dad flew in B17’s toward the end of the war (the war ended just after his 16th?? mission), and he never even saw a German fighter during that time. I know less about the Japanese but wouldn’t be surprised if the same thing happened in the Pacific theatre.
Sixth, IIRC the P38 turned a heck of a lot better than the Bf-110. Apparently the Hurricanes and Spits could make short work of the 110’s, and at some point the Germans started using 109’s to fly escort for the 110’s.
Seventh, boy am I long-winded!