That may be true, but the cost of grading them in any fair way would be prohibitive.
The way essays are graded now (or last time I checked, in an optional SAT writing test) is a disgrace.
They have to hire a bunch of temp workers, probably teachers making a few extra bucks. They give them a sample of a grade-5 (best) essay, then give them all an enormous pile of students' essays. They can only spend maybe one minute on each test.
Also, the model essay they provide reads like a bunch of nonsense written by someone who doesn't have a clue what good writing is, but knows how to snow his teachers.
I can write a piece of junk right now they would consider good writing:
While many experienced beer drinkers prefer Miller Lite due to the fact that the taste is superior, there are also those who elect to consume the beverage because of the lesser amount of filling sensation it provides. To illustrate and better illuminate how this apparent dichotomy can lead to both increased consumption as well as interpersonal conflicts...
(and so on, I think you get the idea.)
I agree that you and I could do a good job of grading the students' essays. But to be fair, I might need to take a few minutes on each test. And then we'd have to get together on each test to see if we agree. No doubt there will be many essays we'll give different grades to, and probably some that we'll disagree by a lot. What's fair to do, just average the two grades?
But the biggest problems are these:
This isn't like one teacher who has maybe thirty, or maybe one hundred essays to grade and a few days to do it. More like there's a pile of a million essays to go through and one day to do it. There's no choice other than flying through them as fast as possible.
The other problem is that after a few hours (actually probably much less) of reading essays, my brain will be so sick of them I'll never want to see an essay again. There's no way I could fairly grade any essay after I've just read a few hundred in a few hours. (Especially not at the crummy wages they pay.)