It’s because human nature finds works more satisfying than faith. It is easier to point to our good works, however biased that view may be, and prop it up as sufficient for eternal life. It also demonstrates that nothing has changed since Paul told the Galatians they were frustrating grace by going back to legalism.
We received grace, not because we deserved it, but because God gifted it to us. The moment we look to our own works as something good enough to gain acceptance before God, we are frustrating God’s grace and undermining our faith in Jesus. He is the only one on whom we can rely for eternal life.