State Standardize Testing

Students in public schools have been taking standardize state test for a long time now and the question is what have they proven?  During the Atlanta school scandal I found out that all this test do is encourage teachers to do the wrong things.  Teachers have to depend on students to pass these test in order to keep their jobs and also to maybe receive bonuses and other perks in the school system. These test though have not proven anything but that some students test well and some students do not.  It has also proven that teachers will go to far extents in order to keep a job in their profession. I say though if these are the only reason these test exist why do we still have them? I see private school students not take these test and still have high ACT scores and a high acceptance into college.  So my question is to you today why do state standardize test still exist? and why are private schools do so well without them?