Hey, Gradescope dev here. What detaro said is on the money -- we're able to group identical short-answer responses so that they can be graded in one shot. It's not necessary to analyze the answer content for this.

Many (though certainly not all) of the instructors using Gradescope are teaching CS or Math courses with heavy enrollment. So each exam will have many submissions (even 1000+), and each submission will have a lot of short answers. Marking each one on its own is tedious, but until recently it was the state of the art for paper exams.

Instructors can and do grade essays on Gradescope, and are able to save time. But in that case the savings comes from being able to create rubrics on the fly, to change point values without re-adjusting every single marked paper, to grade across questions rather than across exams, to publish grades without having to type them all in, and so on.

There's a lot of grunt work that goes into grading, and it doesn't need to be the case :)