Also because when you are in separate cars, you can't get sick from, or harassed by, or groped by, other drivers. If another driver is bothering you, you can usually just speed up or slow down to increase distance.

To get true mass adoption of public transit in the US, as the parent comment said, you need to fix "the poverty and homelessness that make theft, vandalism and antisocial behaviour so common," or at least work around it. Maybe trains with a hundred doors leading to a hundred isolated seating areas would convince people.