For some reason folks have difficulty realizing that motorcycles are not horses.
Chaps were developed to protect the legs of a mounted horse rider amid thorny brush and small trees. The key word is "mounted".
Motorcycle riders generally take interest in protective gear when they become "unmounted" while still in motion. In the most common and least dangerous type of unintended dismount the motorcycle's tires slide out from under the bike, depositing the rider on the road in a feet-first slide.
Note that in essentially any slide feet-first is better than headfirst, and there are only two stable equilibria, face up or face down. Any other arrangement leads to a tumble which tends to break bones.
Most riders, given the choice, would prefer to go sliding face up, feet first. This is exactly the situation chaps don't "cover" and for a guy the face down situation might be considered worse. 8-)
Chaps do have uses, but motorcycles are certainly not an obvious one.