Universities compete for students by offering increasing amounts of stipend money and Stanford's offer was substantial in comparison to other universities. You are fully supported to pursue your education. But you don't know the half of it. First of all, let's review the typical life of a graduate student. Then take into consideration that it may take an individual up to 8 years to finish a Ph.D. - at Stanford, the average is 6 years.
People salivate at the idea of cheap labor.
However, students falter in graduate school for years on end due to the negligence of professors. While the professors are getting paid to train, they do not monitor their students' progress as they should. The amount of knowledge needed to confer the degree of Ph.D. can easily be obtained in 3 years. However, there is the hazing process to which graduate students are familiar, where everything drags on and on. We are slaves.