For the most part the same grad students get tuitions wavers, which if accounted for, would up their income for what is part time work significantly.
On a PhD you do your own research. You aren’t working for anyone but yourself. You may do some teaching but that’s just an optional part-time job.
Something tells me you have not been through PhD, or are have not interacted with other PhD students. You absolutely work on what your PI got their grant for.
I can't speak to what your experience is, but what I am saying here is typical in US universities.
Graduate school is very much directed? The usual criticism is that it’s completely undirected and you can drift if you aren’t careful!
If you want to have a PhD you need funding. To get funding you need to work.
Then you get to research in your free time.
Source: I’m a PhD student.