Isn't it a USNA thing too? I always see USNA guys rocking the V-necks. Why, especially since you have to wear crew neck on board ship?
Story I heard when I was a mid that a supe's wife way back in the day was tired of seeing shitty, stretched/worn-out crew necks so mrs. supe asked mr. supe to change it. Voila, mids now wear v-necks.
Whether that story is true or not I dunno, but then again one never can underestimate the power of an admiral's wife!