The issue is how departments are structured. Generally, every grad student is paid by a grant or fellowship of some sorts. However, how that actually works out

The issue is how departments are structured. Generally, every grad student is paid by a grant or fellowship of some sorts. However, how that actually works out varies a lot by discipline. In STEM, well, really hard science, engineering, and CS, you and your advisor are tightly coupled. PhD students do the bulk of the hard work, while the advisor acts as some combination of directing long-term research thrusts, managing the capabilities of the lab (both in terms of human and equipment), and a veteran with expertise that can chip in when they have time. How that balance shakes out depends on the advisor, field, and group members, but they often do some amount of each in order of appearance. Most PhD students I have known are thus funded through grants their advisor wrote with little to no involvement from students directly (the second most common source is those funded through a few programs like the NSF GRFP, but in the cases I know, their advisor had money lined up for them in the event they weren’t awarded the grant). The application and reward both break down funding in terms of students, equipment, discretionary funds, etc., and are often large enough to fund multiple students. Even if a professor didn’t write much of an article or do the bench work, their name goes on there for guiding the overall project and procuring the funds (and hopefully at least giving revisions on manuscript drafts). Very big schools (e.g., some in California) have high drop out rates because they admit more students than professors can fund and the weaker ones get cut because they can’t find an advisor to take them after their first year or two. Smaller schools tend to have professors more involved up front and admit students only if at least one professor is interested in them joining their group, so the filtering happens much earlier. In the humanities and, from what I’ve heard by friends in the social sciences the dynamic is very different. Your “advisor” and you interact very infrequently because there is no research group being run. They might give you some high level advice but that’s it unless you seek more out. Your money, then, needs to come from an external fellowship to which you applied or a departmental fellowship. I know at least one department whose students are all funded via the interest on a massive donation made decades ago for this explicit purpose (but only for students in that department, the money can’t be used elsewhere). For those students in that prestigious department, life is easy. For students at famous schools, the department often has more fellowship funding and they are more competitive when seeking external grants For students at less famous places, resources are scarce.