The problem is that cars are needed for many people to get to where they need to go. In my case, if I didn't have a car, I wouldn't be able to get to many of my doctor's appointments and grocery shopping would be extremely difficult.
There are buses where I live, but they don't go to where my doctors are. And when I do my weekly grocery shopping, I can have 8 full (reusable) bags of groceries. Even if the buses went to the stores I go to, carrying all these onto and off of the bus myself would be extremely difficult.