That's because the blaming process if pointless, and doesn't help anyone. If there is to be a point to performance appraisal it should be getting manager and employee working together to have everyone get better. They do that because it's faster than doing it right. By the way, if you have two people rate the same employee, the chances of them agreeing are very small. Say it to yourself over and over.
Ratings are subjective.
Rating forms are subjective. Rating forms are not behavioral. Lots of managers do this. They conduct appraisals so long as they have to do so to justify or withhold a pay increase. When staff hit their salary ceiling, or pay is not connected to appraisal and performance, managers don't bother.