That was my thought, too.
People not only don’t understand grace, they don’t understand forgiveness either.
And I think I know the reason. It is a strictly human trait - PRIDE. It is something God says He hates:
Pride has kept many people from accepting Jesus Christ as Savior. Admitting sin and acknowledging that in our own strength we can do nothing to inherit eternal life is a constant stumbling block for prideful people. We are not to boast about ourselves; if we want to boast, then we are to proclaim the glories of God. What we say about ourselves means nothing in God’s work. It is what God says about us that makes the difference (2 Corinthians 10:18).
Why is pride so sinful? Pride is giving ourselves the credit for something that God has accomplished. Pride is taking the glory that belongs to God alone and keeping it for ourselves. Pride is essentially self-worship. Anything we accomplish in this world would not have been possible were it not for God enabling and sustaining us. “What do you have that you did not receive? And if you did receive it, why do you boast as though you did not?” (1 Corinthians 4:7). That is why we give God the glory—He alone deserves it. (https://www.gotquestions.org/pride-Bible.html)