Because holding grudges literally eats you alive. You're also confusing forgiveness for anulling everything the person did wrong.
I can forgive my parents for the pain and abuse I had to endure growing up around them. I do that for myself 'cause at the end of the day I can't do anything to change what already happened. What I *can* do is work with myself to heal from all of it, and part of that is learning to let go.
I have a very touch 'n go relationship with my mother, but my father I haven't spoken to in about a decade now.
Forgiveness isn't just for the person who wronged you. You also don't have to keep someone in your life even tho you have forgiven them. It has been medically proven tho that constant stress, anger and anxiety shorten your lifespan and can lead to debilitating chronic illnesses, too.
In the end, it's almost like you're continuing to let the people who wronged you in the past to keep harming and controlling you.

Ok, why do the ones who went thru hell are demons? just felt unjustified, especially saeki, yes, he held grudge but he was sold off, overall that anger, grudge is a sin no matter what the issue behind was???? I still can't digest the fact, 'forgive all' concept
(ノ`Д´)ノ彡┻━
I follow 'don't forgive just forget' we shouldn't give them the power of forgiveness over our feelings as time passes they will eventually forgive themselves and live without regret, then why should we harrass ourselves with burden of forgiveness? If u do wrong, either correct it in ur actions or let us just forget this relationship existed.