Because he craves forgiveness.
If he can be forgiven by you, the victim, he might be able to forgive himself too.
Hence symbolism and celebration.
The hard truth is this:
Forgiveness may never come. And sure as hell he should never forgive himself for that behavior.
Understand it yes. Healing the flaws yes. Forgiveness to your betrayal choices never. He can become the man who would vomit and cut his dick off rather than cheat on his wife. No questions, he can absolutely do that and be proud of what he’s accomplished, with his head held high.
To become that man you also need no mercy for the version of you who did that. Not for the trauma but no mercy for the behavior and choices.
Yes it’s a stain on your soul, your life, and will never go away. This sucks.
But so we can say about your being betrayed, you had no choice you were only willingly and intentionally abused.
And that wound has been forced on you for your life, scars or not, staying or leaving for someone else, we will carry it until the day we die.
That’s the kind of "gift" our partners have given us.
Only complete oblivion of the abuser might reduce it to almost zero, but some impact will always influence you.
If the guilt sucks, this sucks an order of magnitude worse. No comparison.
Love can return, wounded but alive. Life can maybe get back to decent or even good, happiness is not impossible.
But as you will always carry his taint he should own it and stop worrying about what he wants you to feel. He just counts himself lucky you love him enough that he can lay eyes on you still, after taking everything from you and then some more.
Time to man up mr. you sure can, and find a reason to be proud of yourself. Don’t cling to your wife to feel better. Get better yourself. Then when you’ll get her smile that’s your reward, because you know you are finally changed.
And that’s just better than anything you ever had before.