Because we don't know when He is coming back to destroy this world and create a new perfect one. It could be tomorrow or in a thousand years. And if we are living in sin when He returns then He will not accept us into heaven. Also if you had read thoroughly you would have noticed that it say to repent and turn away from your sins and He will forgive. Sure he'll forgive you when you mess up but true cleansing forgiveness begins with you making an effort to turn away from your sin and not do it again