I once got a C- on an essay, but for extra credit we could submit that paper to a state essay contest. I was a state finalist in that contest, with my only revision from the class-submitted version being spell checking Britan->Britain.
I mean, few things in life can be graded objectively so that's not really the best standard to hold education to. There's a lot of value in learning and practicing the skill of taking in information and then rearranging it into your own words, creating and supporting arguments from the knowledge you've gained.
But the subjectivity of it DOES mean that occasionally teachers will need to set aside their egos. Those who legitimately want to educate the next generation usually can do that, but some just enjoy having power over children and absolutely will not units forced to do so (and doing so will just create a new grudge against that student).
Well of course nothing can be graded 100% objectively but maths for example. Your answer is either correct or not. With essays the teacher can just decide they don't like your argument and you get fucked. I my 4 years of secondary school none of the 8 guys in my class ever got the best possible grade on an essay. Even after we got a less demanding teacher the grades barely improved.