It's fair to assume that Michael Phelps (Olympic gold medal swimmer) won't drown anytime soon, but it is not fair to assume that skateboarders won't hit their heads. Skateboarders can be ejected from their boards for reasons besides a lack of experience or skill with skateboarding, such as rolling into an unexpectedly large crack or rock on the ground, causing the rider to fall on or run into an object, such as a brick wall, metal gate, or concrete planter box.
But consider also cases where a skateboarder's skill level is relevant, such as when they're trying new stuff — new techniques or applying old techniques to new obstacles with unfamiliar dynamics. (Which one could argue is one of the times in which skateboarding is its most satisfying.)
To sum this up, to wear a helmet while skateboarding is not to be Michael Phelps with water wings.