Why would a wife forgive her spouse infidelity if she never did anything wrong to their marriage?

One possible reason is to heal the relationship. In order to move past the hurt, you need to forgive them, but not for their sake, for your own. Meanwhile, you still have to decide whether or not to stay in the relationship and if your partner will change their behavior if you do forgive the infidelity.
Another possible reason wives forgive infidility is financial security. Often women do not have the career opportunities or the financial resources of their spouses.