If I am doing the speed limit, why does it matter which lane I am in and why do you need to pass me?
First, because it's the law in most states to stay right if you are moving slower than the flow of traffic.
Second, it's common courtesy. Speed limits are arbitrary numbers, and most people drive to a speed that they are comfortable driving at. It's not your job to police the world, and doing so causes more problems than it solves.