In the story of the rich young ruler (Matthew 19:16-22), why does Jesus not elaborate when the guy says he has kept the Commandments? None of us have.

Matthew’s account of Jesus’ encounter with the “rich young ruler” goes like this: Now a man came up to Jesus and asked, “Teacher, what good thing must I do to get eternal life?” “Why do you ask me about what is good?” Jesus replied. “There is only One who is good. If you want to enter life, obey the commandments.” “Which ones?” the man inquired. Jesus replied, “‘DO NOT MURDER, DO NOT COMMIT ADULTERY, DO NOT STEAL, DO NOT GIVE FALSE TESTIMONY, HONOR YOUR FATHER AND MOTHER,’ and ‘LOVE YOUR NEIGHBOR AS YOURSELF.’” “All these I have kept,” the young man said. “What do I still lack?” Jesus answered, “If you want to be perfect, go, sell your possessions and give to the poor, and you will have treasure in heaven. Then come, follow me.” When the young man heard this, he went away sad, because he had great wealth—Matthew 19:16-22. This moment must have been quite impressive; Mark (10:17-27) and Luke (18:18-27) report this incident as well.
As to why Jesus did not call this young man on his seeming arrogance at declaring that he has kept the requirements of the law, Jesus actually does something similar to what He did with the crowd accusing the woman caught in adultery (John : He lets him expose the truth to himself. In quoting from the Commandments, Jesus is showing both God’s high standard and man’s complete inability to meet that standard. But the guy doesn’t get it. He is sure he actually is pleasing to God, but he’s still not sure. And the fact is, he is a sinner: He loved both himself and his possessions more than God, and certainly more than any neighbor. So Jesus confronted him with the deal: Sell all your stuff—it’s just getting in the way of a relationship between you and the Father—give all the proceeds to the poor. Then come and follow Me, and that eternal life he longs for will be his. The point Jesus is making here is not that riches are bad or that poverty is good. The point being made is the absolute necessity of being willing to surrender our lives completely to Jesus Christ. Anything less will not do. The price Jesus asked was too much for this man, and he walks away. Jesus let His requirement of surrender speak to the man. While he might be able to argue his success with the commandments, he couldn’t argue his way out of Christ’s “unconditional surrender” terms. And even though it wasn’t planned, it’s interesting that questions arise about the “rich young ruler” and the “woman caught in adultery.” Remember what Jesus said to the Pharisees when they questioned His hanging out with the so-called tax collectors and sinners? They couldn’t understand why He would do that. What did He say? “It is not the healthy who need a doctor, but the sick. I have not come to call the righteous, but sinners”—Jesus Christ, Mark 2:17. These two incidents really illustrate strongly what Jesus’ words look like.