And what happens if they don’t want to do it?
https://biosciences.uchicago.edu/content/teaching-assistant-...
> All graduate students are required to serve as a Teaching Assistant in at least one course for academic credit before the Ph.D. degree is awarded. Appropriate courses may be undergraduate, graduate, or medical, but must be in the Biological Sciences Division (exceptions may be made for students in the Biophysical Sciences program).
it works out really well for the student. instead of working a regular job during the year you get to do something that benefits your career, you build a relationship with your professors and it pays more than if you were working at the local grocery store.