[1] When the war began the Germans debuted a number of Marvels that no one had suspected existed. The first hint that the Germans were developing, in an organised way, a group of supermen came during the Spanish Civil War, when the superpowered Condor Legion wrought such havoc and damage on the Loyalist lines and troops and on the Spanish civilians. The news media in America and on the Continent followed the wishes of the various governments and played the story as if it was a horrific aberration; the last appearance of a group of Marvels anywhere near the size of the Legion had come during the end of the War, when the Freedom's Five and the Doughboys, led by the American Eagle, had confronted the Junkers in Belleau Wood and afterwards. But that had been a once-in-a-lifetime event (nobody believed the rumors about what had happened in London and on the Continent around the turn of the century), and the idea that such a thing might occur again was dismissed as unbelievable by the press--both those who genuinely thought it impossible and those who knew otherwise but wanted to keep their governments happy.
It was only when 10 Downing Street and the White House were broken into, and detailed notes left on the desks of President Roosevelt and Prime Minister Chamberlain, that the thinking about German Marvels changed. It was suspected that a member of the Secret Six was responsible for this act, but no one ever knew for sure, and if the Secret Six actually existed, and one of them had done it, they weren't saying.
After that the attitude of the American and British governments changed. [2] The American Office of Naval Intelligence and the British MI-6(?) began sending agents--paranormal and otherwise--into Germany and Japan and Italy. What they'd discovered had horrified them. From somewhere--no one knew where--the Axis powers had gained or developed the technology to produce their own superhumans. Powerful ones, too, enough so that they might be a match for the Americans and British Marvels. After all the information was gathered, however, the Allied governments still had a (somewhat shaky, true) confidence that between their technology and their superiority in numbers, they could beat the Axis supermen.
Then came the invasion of France, and the Allies realised they were outnumbered and outgunned. And after that came the invasion of Russia, when the Allies realised that they were outnumbered 5 to 1 and, on the face of it, had no hope against the power and numbers of the German Valkyries.
[2] Only the French were let in on this secret, and they were condescendingly dismissive of the idea. "Not possible," they sneered, and "If such a thing was true, we surely would have heard of it by now, non?" and "Stick to your games in America and India, and leave the Continent to those who know it best." The French had their own small group of irregular Marvels, Les Chasseurs, and felt safe behind the Maginot Line and with their Marvels on the job. The events of 1940 painfully disabused them of this assumption, but by then it was too late.
[3] Savage himself wasn't sure why he was so dead-set against the rest of the world being able to use the the weapons and aircraft and technology that he created; introspection was not his long suit, and his emotional development had been stunted by the early death of his father and the ever-present knowledge that his task--saving the world from itself--would leave little if any time for a personal life. He'd never had girlfriends or real playmates, growing up, and the only woman he'd ever really loved, who'd loved him back, was Pat, and now she was dead.
So Clark "Doc" Savage wasn't emotionally mature, and had a number of sadnesses in his life, and these discouraged him from introspection, which is why he couldn't have articulated his opposition to sharing his advanced technology. If he could have, however, it would have been a statement along the lines of "The Allies can have the molecular rearranger and adamantium generator and heat rays if they need it; but such items are too powerful to be given to ordinary people, who might be corrupted by it. Only extraordinary men and women, whose morals and consciences can be relied upon regardless of the temptation, can be trusted with such items. And I'm the only one of those that I know."