It's not just lab work, I TA'd for physics and students would come to TA discussion sessions to get a more detailed lesson on the material covered that week sin

It's not just lab work, I TA'd for physics and students would come to TA discussion sessions to get a more detailed lesson on the material covered that week since often the professor is just repeating the contents of a textbook. Also, many TAs would spend many additional hours in tutoring and office hours, giving individualized attention to students, and this workload adds up. TAs also grade quizzes, final exams, create questions for problems, etc... The professor often does very little work and leaves it to the TA, at least in my experience as a TA for physics courses. It's a lot more than just maintaining a lab.