Yeah, this is the #1 thing people get wrong about STEM grad school. Not only is tuition 100% covered, you get a stipend that's usually 20-30k per year depending on local cost of living. Masters is completely covered too if you go in to a program that's expected to continue on to a PhD, and will as likely as not be covered even for a plain masters in some pure science fields.

Non-STEM too for Harvard and Yale, I have friends who are art history PhDs at both places.

However, it's a long slog without a big money payoff after graduation, in fact it is better to be a student in most cases.

The stipend is even north of 30k in STEM at places like Harvard and MIT.

It's even north of 30k at places like John's Hopkins in super cheap Baltimore (and this was more than 10 years ago when I left)

Could be a private school vs public school distinction. UC stipends were < 30k in the early 2010's.

Perhaps, but our stipends were determined by NIH training grants.

It was <20 at UCSB in chemistry then.

It really depends on how well the program/department is funded. Computer Science usually is. I was in a program (Environmental Engineering) where most masters degree students had to pay full price. Grants or graduate student positions wouldn't kick in until year 2. Even then, full tuition was never a guarantee. This was UC Berkeley.