 because they've died, they hurt you when years ago, they've since passed on, so they can't make amends, even if they wanted to, they can't because they're not here, or maybe you don't even know where they are anymore. So even though you're still hurting for it, they've moved away, they've gone somewhere, so they can't repent. If you don't do unilateral forgiveness, then you'll stay hostage to something that can never be fixed.