Because the American people don't really choose who's on the final ballot. For all we know FDR in 1944 was considered "just ok", but was better than his opponent.
Then that was the fault of a lack of a challenger within the Democratic nomination. And if they really wanted to, more delegates should have voted for Harry Byrd:
Given that Americans tend to reelect presidents during wartime, that makes the claim that FDR was somehow unpopularly "just ok" to be even more dubious.