how is that a useful metric for anything, really?

Look how many extra programs pull people into community colleges, and look at the difference in accessibility between community colleges and universities.

A stay-at-home mother isn't going to be eyeballing Stanford as a viable way to get her nursing degree with night classes. In the same instance, a stay-at-home mother who is using community college in order to get a degree necessarily has a lot more difficulty in managing her time and money than a university aged adult being sent to university by their parents with far fewer personal responsibilities and burdens.

The stay-at-home mother will necessarily have higher drop-out rates just simply from things that exist as burdens in their life from the get-go. As will the displaced worker who is being funded by the state for re-training at their local colleges.

University life is designed to reduce burden on the student in order to facilitate their learning. Comparing metrics like drop-out rates between universities and community colleges is wholly unfair. They both result in degrees, but they serve (mostly) different communities of people.