I would assume that a good chunk of students in poverty simply don't have a device that works well for consuming books on.

If you don't have a tablet or laptop, just a phone with a small screen, I can see people saying z-lib isn't helpful for them. That they'll just use physical books at their library. (And students without computers is definitely still a thing, that's why computer labs still exist.)

I can definitely imagine a lot of undergrads who would assume that if a book isn't available in their college library then they'd never need it anyways. (Rightly or wrongly.)

And remember that so many textbooks now contain a mandatory online component where assignments get submitted and tests are taken, so you're forced to buy it even if z-lib has it. (I'm not defending that... just explaining it.)