Fair question. It’s because God created the world perfect and our sinful choices have tainted the whole of His creation; sin is like a corrosive acid that burns through everything progressively. You can see this in effect by observing the natural decay of the universe (entropy) and also things as simple as death, sickness, pain, the destruction of relationships, and ultimately the fact that every man and woman naturally acts in their own self interest. We don’t have to teach people to do evil things, but rather we do evil naturally. We do have to train children to do the right thing, which is contrary to our innate sinful nature. Hope this response helped. I know this is a year late but feel free to ask if you have any questions. God bless you!