AFAIK the wages PhD stipends are pittances and make it very difficult for someone without money to survive, especially for non-STEM programs. If anything doing a PhD makes you worry more about money because you can't earn enough to survive
In CS, stipends are below median income, but get close to median when students do the occasional R&D internship.
The main CS grad students I've seen struggle are those with families (ex: someone I respect had single income supporting his wife and ~3 kids!), big spenders, and those ignoring (many) corporate R&D internship offers. For everyone else, it's a nice life, and not hard to come out with savings. Likewise, if you're used to living a beyond-median-income lifestyle, e.g., software engineer returning to grad school, it may be hard to go back to living like the rest of America.
As others noted, much harder is the opportunity cost. Why would a student turn down an offer for 3X+ the US median income doing ads etc at some FAANG/VC co for a long, below-median, and relatively thankless & isolating phd experience? "The best minds of my generation are thinking about how to make people click ads".
Given a lack of career path through the PhD program, these programs are likely to attract individuals who don't need a career in the future.
To be fair, I went to a highly prestigious private institution where funding was guaranteed, and while pay was shit, it was enough for a single person to live reasonably comfortably (forget saving, of course). Some public schools I got offers from like Berkeley couldn’t even guarantee funding.
None of my PhD student peers had billionaire parents - but they almost all had parents who were reasonably well off. None of us had to rely on handouts from our parents, and none of us had loads of disposable income - but we all had a safety net.
The undergraduates who grew up in poverty and went to university to improve their lot in life? The £40k job offer represents financial security. It was rare that they'd forego that.