huh ? what's your education system ? in europe it's generally 3 year bachelor, then 2 year masters, then 3 years phd so when you have classes during your phd hopefully they are different that the one you get during your masters
Some courses are more basic, and some are more like collaborative research seminars. Advanced students gravitate towards the latter (and typically take fewer courses and/or audit more of them), but a first year with the right background can still take them. Conversely, an advanced student might take an introductory course in an area they lack background in to broaden their knowledge.
There also are terminal M.A. programs, but they're less common, and the majority of students who do Ph.Ds at prestigious institutions don't do one.
It is not ideal because many of the masters students are coming from a non-CS background. Some of the graduate level CS courses I have taken were easier than 4th year level undergrad classes.
But this is Cornell, which I assume is like other research-oriented US universities. You have a 4 (or maybe 5) year undergraduate, leading to a Bachelors and then apply to graduate programs. The graduate schools may have specific Masters tracks, but not really any general Masters program. Instead, you spend the first year or two taking graduate classes until you either satisfy the assorted requirements or pass an oral qualification (They still have those, right?), after which you focus entirely on research, publications, and your dissertation defense. If you leave school before defending, you get the Masters as a sort of consolation prize---most US PhDs in my experience don't have Masters.