[mmanfrin]

Because of how we stand. We want to be able to reach/see higher, which means stretching upwards. When you are lower/your chin angled down, there is near continuous bone coverage, but when you are up/angled up, that area stretches.

Allows your head to pivot up and down from the hinge of the top of your spinal column, but also allows for the front/variable length bit (the front of your neck) to be protected when needed.

e: Expanding on this a bit, the only other animal that comes to mind that stands like us that I can think of is the Meerkat. Why don't they have chins? Maybe it's that they only stand in situations, not for movement; or maybe it's that they aren't in situations where they can/have to fight back vs predators (not that they don't have predators, but that human vs wolf is a bit more even than meerkat vs lion, more of a chance for the human to prevail and more of a chance for a human with the optional-neck-guard to eek out a win where a chinless compatriot wouldn't).