Would you want to be strapped to the outside of your SUV in a rollover? For the same reason, motorcyclists don't want to be strapped to a motorcycle. Kids have to wear helmets in most states, until they are 18. I won't waste time debating the benefit vs. detriment aspects of helmets, except to say the only time they are really needed is if you are in an accident, and sometimes, they can kill you then. If not wearing one helps avoid an accident, the helmet is moot.
While bicycles and motorcycles each have two wheels, a larger road motorcycle can weigh upward of 650 lbs.
The dynamics found riding motorcycles are different from bicycles unless you are riding the bicycle over 15-20 mph, and those low speeds generally are the ones where protective equipment is more likely to actually prevent injury.
Pull your SUV out in front of someone riding down the highway at 65, and the helmet will not prevent the sort of massive trauma, including head trauma that occurs when the brain sloshes back and forth inside the skull. (Think subdural hematoma).
Those forces are why helmets can't protect the body from stresses it was not designed to absorb.
In a vehicle where an airbag actually absorbs the energy, it is a completely different situation.
Add another two to five pounds to your head and drive a vehicle that requires that you be able to look all over to keep from being hit by people who will claim they "didn't see you" and the fatigue level goes way up. Some like helmets, some don't, and the only reason I'd consider one around here now (as opposed to the past) is the serious increase in heavy truck traffic coming off lease roads slinging rocks and mud. It is a pain to try and keep windshields in my vehicles, I don't need a rock in the face.
One more tidbit of data: the most often broken bones in motorcycle/car accidents? Tibia and fibula, followed by the femur, radius and ulna. The skull actually comes in way down the list, below pelvis, humerus, and ribs...collarbones are popular fractures, too, along with carpals, metacarpals, tarsals, and metatarsals.
Whatever...I drove race cars (SCCA) for years and no-one would have considered being in one without belts and helmets.Are they always effective, of course not, but don't try to convince me that helmets on a cyclist are a bad idea.....of course leathers are some protection for abrasions on the body, but your head banging against a curb is well protected by a helmet.