That’s not always true. In the STEM classes I’ve taken, the grading was done by machine. In some cases that mean multiple-choice questions, but in most cases, it was a matter of “write a computer program that does X,” and the programs would be graded by computer.
Multiple choice homeworks sound easy, don’t they? Not so fast. One of the hardest (and best) classes I took was a machine learning MOOC from CalTech, which was allegedly exactly the same as the on-campus course. Each week we’d have a homework that was ten multiple-choice questions. Each question had five possible answers. The difficulty was, the questions were either difficult math problems, or questions like “Write a program that does X. Run it 10,000 times and compute statistic Y. Is it (A) 10. 5, (B) 8.2…” There was no way to guess the answer. Or, “Write a program that does X. Write another program that does Z. What is the ratio of statistic Y from X to statistic Y from Z.” I spent a lot of time on that class each week.