The human brain is encased in the skull, "floating" in a thin layer of cerebrospinal fluid. This acts as a thin "shock absorber" to cushion and resist the impact of the brain against the skull in a deceleration injury.
The brain itself has the consistency akin to jello. A fresh brain laid out will "flatten." Its strength/resiliency relies on its immediate environment and the physics of floating in liquid, leading to its resistance to rapid changes in velocity (deceleration injuries).
The large mass of the human brain, relative to that of the thickness of skull and proximity to the surface of the head leaves it relatively weak compared to some other animals.
I have ridden without helmet for short distances, moving bike from one house to another around the block. But I do realize that even from the height of sitting on a stationary motorcycle a fall to ground uninterrupted with direct head impact is enough to kill. Bike speed is relative, it is the speed and impact of the head vs object when it comes to head injuries and the merits helmets, if we limit the discussion to that.
You're making a BIG assumption regarding the mass of an average human brain. I'm betting 50% of the folks out there have a considerable larger mass of insulating fluid, than brain mass. One passed me the other day on a crotch rocket doing about 95 mph. I refer to them as 'future casket inhabitants'.
These are the times that try men's souls. The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country; but he that stands by it now, deserves the love and thanks of man and woman. Tyranny, like hell, is not easily conquered; yet we have this consolation with us, that the harder the conflict, the more glorious the triumph.
Thomas Paine December, 1776