Related to the advantage of no tuition:
With roughly 90-95% of Illinois high school students attending public schools, they are the default school option in every single district and have been that default option pretty much since the inception of the public school system in this state. Indeed, in the vast majority of districts, they are the only school option.
One might go so far as to say that most public schools have a monopoly on education. If public education were a business like an airline, telecommunications firm, or a grocery store chain, the government would step in (as it has in the past in those sectors) and demand that there be competition in the educational marketplace.
Granted, private schools can enroll from any district within their radii but being that default option for generation after generation of students is an advantage for public schools if for no other reason than habit/tradition (especially if it involves saving money) is a hard thing to break. Families are IN THE HABIT of sending their kids to public schools. They are IN THE HABIT of not paying extra for education. Families who have sent their kids to public schools for generations find it an easy decision to make where to send their kids to school because they really have no choice in the matter.