There is nothing wrong with many public schools, and plenty wrong with others. Naturally, it has a lot to do with WHERE, in the U.S., you're referring to and what sort of issues they need to deal with. Some school systems, do an excellent job in most areas of importance. I think it's fairly public knowledge, for the communities to judge for themselves, and then make the choice they are most comfortable with, for their children.
As long as private schools are available as an option, parents will express their preference, which they have a right to do. If the tuition is not a financial difficulty, I guess it's fine. I have known families to scrimp and sacrifice, in order to send their children to private schools, because they are so dissatisfied with the particular public school system. It's all a matter of choices and availability.
I'm actually surprised, or perhaps better said, impressed, by the number of parents who "home school" their kids. Although I am in favor of home-schooling, I personally would not have done it. I'm capable, but for my sons, it would not have been a wise decision. I also feel Our public school system is well above par and they all had excellent educational opportunities, along with the many extracurricular activities, so important to my athletic, musical and artistic sons. Yes, I do brag...obnoxiously. That's ALSO a choice! LOL.