10 points
Because they’re 2 different types of vehicle
You’re inside the moving vehicle and should you not be attached to it you’ll bounce around the inside or go flying out the window in a crash
A motorcyclist is not contained, and thus it’s dumb to attach them to the dangerous and heavy metal should a crash occur. We instead require protective gear (unless you live in a shitty state) so you get less injured if you go flying
