They aren't just doing research, they have several classes to manage, often PhDs and graduate students to supervise, and exams happen 2-3 times in a typical sem

They aren't just doing research, they have several classes to manage, often PhDs and graduate students to supervise, and exams happen 2-3 times in a typical semester of 15 weeks. Add on that as exam periods approach, professors also tend to get pretty busy in office hours with students who need help. Combine that with the student's own situation of having 4-5 classes to deal with. Sure, students have a reasonable expectation of having their grade mean something, but it's obvious that oral exams are not the solution. It's also kind of hilarious that in this thread, about a professor falsely accusing an entire class of cheating, you seem to imply that there is enough cheating going on for grades to be meaningless.