You've piqued my curiosity. Why do you believe this to be the case?
McArthur was always a political General in it for himself (and screwing Aussies)—which of course made his ruling Japan the biggest success of his career. He ran Japan through the local elites, he protected the Emperor from consequences by throwing Tojo under the bus, he basically treated Japan pretty well given the circumstances. Obviously the long-term consequences of letting Japan off the hook, the muzzled press, and only partial deconstruction of the zaibatsu all came back for revenge—but the short-term was that Japan quickly rose to be an industrial power and it was able to do so because of how lenient overall McArthur was, and how good he was at the political side (Edit this, and various minor changes: not to take away in any respect how brilliantly Japan’s government exploited that turning point to turn Japan into an incredibly successful country in a shockingly short amount of time).
There are plausible replacements that might do as well as McArthur, but I don't know internal American military politics circa 1945 so I don’t know who’d be likely. McArthur also saved millions of starving Japanese through prompt and large food delivery network infrastructure—Japan was on the edge and sometimes over into famine conditions for years after the end of the war.
Then of course the Americans abandoned their plans for Japan because Korean War / communism (incidentally also a key element in sinking left-wing opposition in Japanese internal politics for a while) and left their societal reengineering half-finished because now they needed Japan as an ally. So that didn’t help much in long-term matters but also meant America now needed/wanted a strong Japan to fight the USSR and because of McArthur the Japanese elites were cool enough with Americans to get on the Cold War train.
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