In medical school, 120 hour weeks really only exist for 4th year med students who are on sub-internships in surgical specialties like neurosurgery, orthopedic surgery, general surgery, etc. Students in those fields do 3 or so month long sub-i's at hospitals they want to do residency at. They basically function as month long interviews for those programs, and are also used to get letters of recommendation from prominent faculty in the field. People who are interested in those fields know about the hour requirements on sub-i's and in residency, and self-select in a way.

The reason resident physicians work so much is because residents make $60k working 80+ hour weeks, and the PAs or NPs who would be willing to cover the floors at night would make $150k+ working 40hrs/wk. Residents also have no collective bargaining power because of the residency match system. You can't easily leave one program and go to another because you don't like the working conditions, for example. If you want to leave, you're more or less blackballed unless you're switching fields. And you don't want to speak up about the conditions because you've got $300k in debt and a degree that's useless until you finish residency.