"How fast you're travelling is mostly irrelevant, what matters is how hard your head hits the deck. The street surface is perpendicular to your velocity, so your head generally receives an impact no harder than if you had just fallen over while stationary."

Out on the street, there are plenty of vertical surfaces you can hit (lamp posts, trees, curb of the road, or just irregularities on the road surface)

That is why motorcyclists wear helmets, while pedestrians do not, although their center of gravity is higher than that of a motorcyclist.