God was so perfect he had to wipe out all that he created and then basicly star over. So how can we still be sinful sinners? Makes no sense ,unless if you look at the fact that Noah was hardly perfect.These stories make no sense or have any reason.
This is where the concept of original sin comes into play. Basically because you are a descendant of adam and eve (aka human), you share the burden of their crimes. It's their way of saying that even if you are a really good, selfless, and caring individual you are still guilty and therefore "need" to be saved.