Bikes dont have airbags, restraints, or a large cage of structural metal surrounding them. If you are on a bike, your only protection is what you are wearing. With that in mind, wouldn't you want to wear something to protect yourself when moving at higher speeds? Even a speed of 10mph can be fatal if you fall off and hit your head on the ground. You cannot fall off or out of a car if you are properly wearing your seatbelt, and the airbags and structure of the vehicle are your immediate protections.
Basically, helmets in cars aren't mandatory and don't make sense to make mandatory, because there are already safety precautions in cars. Bikes, whether manual or motorized, do not offer these or any protections.
Please tell me what exactly a helmet in a car will do for you, unless you are travelling well over 200 miles per hour? Seatbelts already hold the torso in place, preventing one from slamming their head into the steering wheel, dashboard, or windshield, and the airbags already absorb the energy and arrest the unrestrained body parts, such as the head.
You would have to be travelling fast enough to outpace the airbags, which typically deploy at around 200 miles per hour. You wanna know why professional race car drivers wear helmets? Because they don't have airbags.