I have, though that may be due to my ever-inquiring mind! I wonder about everything. I was a very annoying child.
Early American sweatshirts – from about the 1940s – were used purely for sport, and so they had absorbent webbing around the neck, which is where that v-shaped patch of sweat usually starts. This v shape was made of entirely different (absorbent cotton) material from the rest of the sweatshirt. There was no lightweight moisture-wicking sports gear then so soaking up sweat was the best you could hope for. And that is what that v did.
This webbed material also made the neckline more stretchy so that it was easier to get the sweatshirt on and off.
Later on, as athletic gear developed and became more high tech, this v became purely decorative – a patch of cotton stitched into the sweatshirt, or just stitches in the regular material.
Fashion being what it is, when vintage is in, there are v shapes in sweatshirts and when vintage is not in, there is a regular round neck with no detailing around the collar.