Aren't graduate students there in order to get credentials that help them land all those better jobs outside of academia? I think you're imputing hiring pressure that they're pursuing, not experiencing.

Not necessarily; I myself and a lot of my friends in academia are here because we really do care about the research we are doing, and believe what we are doing here is really going to impact society in 10-20 years in the future. I'm sure there are some people here who are only doing a Ph.D. to get a better job afterwards. But at least for myself I know I rejected a job that has a 5x base salary (even without counting the RSU) because I really believed in what I wanted to do. If you take the opportunity cost in account, it really doesn't make sense in 100% of the cases to get a Ph.D. first to just go into the "better jobs" in industry afterwards.

That's absolutely fair. I really should have narrowed my (rhetorical) question to the group of students who will eventually "leave for better jobs" outside academia.

Ignoring professional degrees like MBAs, MDs, and JDs, you either go to grad school to do research and/or teach in academia or to learn very specialized skills and go into industry. For the most part, there are usually less specialized roles in industry for people with bachelor's degrees.