This might be the case for CS PhDs, but it certainly isn't for the vast majority of Stanford PhD students in other departments - many of whom are earning less than half the amount you cited. A large percentage of Stanford PhD students work at least one additional job, so the reality is that outside of a select few departments you cannot "focus on it 100%."
I'm much less familiar with the financial model in the humanities, but I don't think your information is correct. (What's your source?) Across the whole university, Stanford's minimum salary for graduate research assistants this year is $12,054 per quarter (four quarters per year), and $12,522 per quarter when serving as a teaching assistant. (https://gfs.stanford.edu/salary/salary23/salary_tables.pdf, and here's what the financial package looks like for a PhD student in, e.g., education: https://ed.stanford.edu/admissions/financing/doctoral)
I'm definitely not telling you to go get a PhD in English/history/education/theater for the money, but if you did, my understanding is that the salary you could expect here is around $48k-$49k a year (+ health plan). I don't think any PhD student is earning less than half the amount I cited.
When you say "A large percentage of Stanford PhD students work at least one additional job," if you mean TAing, then yes, that's true -- I think every PhD student has to TA at least a few times, and more depending on how well-funded their program or advisor are and whether they have their own fellowship. (Regular faculty also have to teach on top of our research responsibilities.) But if you mean some outside-Stanford job, I don't think this is common.
