I don’t think a helmet would be all that helpful. For a person wearing a seatbelt in a modern car with airbags, there really isn’t a lot of hard things to hit your head against.
In these situations, the biggest damage to the brain is something called diffuse axonal injury where rotational forces from the sudden acceleration and deceleration cause injury to the neurons. A helmet would not protect you from that.
Not everywhere in the world. In poorer countries car manufacturers and regulators have perverse incentives to cut costs on safety devices, to be able to sell for a poorer clientele. For example, in Brazil up to 2009 airbags were not even mandatory, and when the regulations finally required it, they were under fire for several years. Today airbags other than the frontal ones for driver and passenger riding shotgun are still uncommon here. See https://apnews.com/article/ac0ec2592da24f2bb6386b58d0db074f
In these situations, the biggest damage to the brain is something called diffuse axonal injury where rotational forces from the sudden acceleration and deceleration cause injury to the neurons. A helmet would not protect you from that.