So the reason this isn't the case, is because there are very simple metrics that tend to highly correlate with essay quality. It doesn't mean the grading-bot is actually evaluating essay quality. It's just looking for properties that are statistically associated with good essays. Remember, at the end of the day as long as the bot's ranking is close enough to the human grader's ranking, nobody really cares about the internal logic.
A very straightforward example is spelling mistakes. People who make spelling mistakes aren't necessarily bad writers. And vice versa, there may be great speller who can't write for shit. But by and large the people who spell poorly also tend to write poorly. Easily detectable grammatical issues, like misplaced modifiers, subject verb disagreement, or inconsistent tense, are also correlated indicators.
A very simple metric is essay length. Especially if its a timed exam. Good writers tend to have verbal fluidity, with words easily flowing to paper. They don't struggle converting thoughts too sentences. So they tend to end up with the most words written down within a fixed time period. By and large the longer a timed essay is, the more likely that its actual quality is high.
Grading bots basically rely on these statistical relationships. They're not measuring anything intrinsic to good writing. But at the end of the day, their student rankings are usually pretty close to that of a typical human grader. In some cases the bot will have a closer ranking to a random human grader, than two random human graders will have to each other.
The biggest flaw here is Goodwin's law. When the test takers become aware of the kludges that the bots use, they can exploit it. For example just dump a bunch of verbal diarrhea with as many correctly spelled words as possible. But even then it doesn't really hurt the bot's ranking accuracy too much. Because the kids who do the most test-prep and learn all the tips and tricks, are usually high-achievers who do well on essays anyway.