I think you'll find there's a wide space between "do nothing to prevent" and "prevents all" and, depending on the sport and helmet design, helmets live mostly i

I think you'll find there's a wide space between "do nothing to prevent" and "prevents all" and, depending on the sport and helmet design, helmets live mostly in that space. Gridiron football has large, quick people starting facing each other, often getting hit from the blind-side, and no helmet is going to prevent all (or even most) concussions in those situations, which is part of why the NFL has (very belatedly) attempted to address things with rules changes. Sports that involve being intentionally hit in the head repeatedly (like MMA or boxing) are going to be much worse. You still see people wearing helmets in friendly sparring matches for those sports because it can prevent freak-accident concussions that happen in such a context.