I'd guess because for a lot of us in the USA, public transportation is insufficient to meet our needs. I'd love to take a train from home to work, but there's no train line that's anywhere near my house. They're building one that'll go near my work, but it's not done yet. Busses are available, I suppose...but the time it'd take to get from home to work or back would be a lot longer than driving takes, even in heavy traffic, given that I'd have to transfer several times.