We are born sinful creatures (the bible says we are evil from childhood).
That might sound a bit extreeme, but concidering even young children will bite, take what they know they may not, lie once they can talk etc, it should be evident. We are simply born with a sinful nature. Now that is not an excuse (once we grow old enough to reason about it), as Jesus calls us to repent, but I guess it is "the reason why".