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
Because (in NL) you're supposed to wear a full protective suit with padding and a helmet, or get a significantly worse ticket and potential loss of head/limb.
I used to think that too until a friend pointed out to me that I might bog down the healthcare system with injuries that could have been easily avoided if I had.
On the one hand, it's a victimless crime, at least roughly. On the other, people have been shown incapable of making the obvious correct decision on their own.