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

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.

Cars should not be street legal. If I can get a ticket for not wearing my helmet, why do cars get a pass?

You shouldn’t get a ticket for not wearing a seatbelt.