Weird theory of mine: The DoD ultimately picks these winning planes based on which one “looks cooler” or basically has better aesthetics. I think the F-35 has better aesthetics than the Boeing X-32, just like the F-22 has better aesthetics than the YF-23. You could say I’m used to seeing the winning designs as “normal”, but I distinctly remember having this reaction before the JSF competition was over.
In any case it's also good military thinking. Sun Tzu already said that the pinnacle of warfare is to gain victory without fighting, and one of the ways to do so is to look so menacing that your opponent does not want to engage. Sleek, powerful looking military hardware has that effect much more than bloated and clunky stuff.