Unlike a multiple choice test where the primary audience is automated graders, the primary audience for an essay is other humans. If even Google and Facebook with their billions of dollars and billions of posts worth of data, still cannot always understand the intent and purpose of written content, what hope do these algorithms have?
If it is cost-prohibitive for every essay to be graded by humans, then they should be dropped from the tests. Otherwise, we are missing the whole point of essays which is to communicate effectively with another human, not just match certain text patterns.