Ya, effort is required for sure, but I think the papers should be marked according to categories that actually can be marked, such as grammar and spelling, factual correctness etc. There is still some subjectivity, but it's way more fair.
I once got an A in a philosophy course where I never even turned in any work. I think the teacher knew I was generally smart and argumentative, and just gave people marks accordingly.
I wouldn't do that today, because I'm more aware of how irrelevant school is.
This is simple human bias if you expect something to be good it will be good, and has to do with how your teacher feels about you.