You are just assuming I, or anyone else for that matter, can use bus transportation.
A bus route has to be convenient for one to use, never mind having the bus schedule coincide with one's transportation needs or even if bus service is available in one's immediate location.
Some people who do own a car do take public transportation. I have a friend who used to drive to the train station to get back and for the to work every day because route I95 was a nightmare to navigate to get to work on time. Also, if one does not use their car to travel to work, they save on car mileage, gas costs, wear and tear on their wheels and other car parts, plus they do not pay higher premiums for those who use their cars to commute to work. Using public transportation while still owning a car is convenient for many people, not to mention it saves their car from racking up thousands of miles, protecting their automobile from being replaced sooner.
Although bus fares may be cheaper than owning a car, bus routes tend to stop quite often, causing people to spend a lot more time to get from point A to point B. Some people just cannot wake up at o'dark thrity to catch a bus to work early enough because they have to get their children off to school before they leave for work.