Why can't outward morality save us?

Outward morality, as exemplified by the Pharisees, is insufficient for salvation because it does not achieve the perfect holiness that God requires. Jesus emphasized that unless our righteousness exceeds theirs, we cannot enter the kingdom of heaven (Matthew 5:20). God's standard is absolute, and all attempts to attain righteousness through human effort are ultimately inadequate, as stated in Isaiah 64:6: 'All our righteousnesses are as filthy rags.' True righteousness that justifies comes only through faith in Christ, who fulfilled the law and provides the necessary righteousness we lack.