First: nobody is forced to get a graduate degree. In many fields (like programming), it's nothing like a requirement to earnings maximization. Since grad students already have degrees, they are also free to work and save until they can pay for fees outright or reduce their loan burden.
I don't have total stats in front of me, but it looks like the Law/Medical/Business schools are ~50% of graduate employment at Harvard. Take it as read that these are worth borrowing for.
The other 50% -- AFAIK there is a significant portion of students on fellowships/scholarships of some kind. My guess is very few people are taking on a bunch of debt for a Divinity PhD.
At this point, it's fair to say that Harvard offers better financial value at all levels for its programs than many public universities offer their in-state residents.