Why is it useless for the sin-bent personality to try to clean itself up?
Because the problem is not merely outward behavior but identity: every person is a son of Adam, born with a sin-bent personality. Self-reformation cannot change that root condition. Only God can do what is necessary, so any attempt by the sin-bent personality to clean itself up is ultimately for nothing.
Les teaches that Paul lays out the root problem plainly: it is not what we are doing, but who we are. We are sons of Adam, and every baby born into the world is in the same place Adam was after he fell. The baby comes in a sweet, innocent little body with its five senses, but lying with that body is a sin-bent personality. That personality can try to clean itself up, but until God does the things which only God can do, it is all for nothing. The soul or personality, along with the mind, will, and emotions, is invisible yet real.