A couple of years ago, I would have agreed with you wholeheartedly. However, I have recently seen instances of lower income areas with great (better than my children's schools) test scores. I don't know what the teachers are doing there, but it's working. People KNOW how to do this the "right" way. Why everyone isn't flocking to these schools to learn their practices is beyond me.
It's a good question. If I had to guess, it's because there's no real repercussion for subpar performance. In the private sector companies go out of business; not so in education.
I also think it's easy to make excuses about the fact that public schools can't control their input; they have to take whatever kids walk in the door.