It seems plausible that as you add more people to the project, the people added later have fewer skills or less interest compared to the ones who are more engaged ones who joined earlier.
Early war designs also had dangerous flaws that had to be corrected, which slowed down construction of subsequent ships, but made them less likely to spontaneously break apart.
Between that and muss less skilled, much less experienced labour producing ships in newly constructed shipyards with teething issues that were also in less ideal locations... something had to give, and taking a bit longer was the least bad option.
The US "solved" that particular problem by keeping its army much smaller than it could've been, precisely to free up more healthy 18 year olds for factory jobs.
Between the British Empire, the various Chinese governments and Soviet Union, the Allies had plenty of manpower, but all that was meaningless without US munitions, US equipment, US food, and US ships to get them where they were needed.
