It's hard to say why there is so much churn. The churn related to changing schools pretty much can't be about pay because you get paid the same amount no matter where you teach unless to change districts, and even then you might only get a couple hundred dollars a year more. But changing schools can get you a better administration or a better pool of students to work with.
However, if you get offered a 9-5 job for $100,000 and you are looking at making $48,000 after several years experience ... you start imagining what you could do with an extra $50,000 a year. Just think of the compound interest!!!
If you can make lots of money, you almost owe it to yourself to get it. But who does that leave us to educate the kids that are going to run the country when we are old? Do we really want to leave that job up to people who can't do anything but teach (so they live and die by the dreaded tenure system) and people who don't have experience teaching (and will soon leave to make money in industry)?
Also, not all states have unions or tenure (my state included) and those states to not have industry-competitive salaries and they do not provide better educations. If unions were the problem, wouldn't we see better results from non-union states? Wouldn't it behove non-union school districts to out perform/compete union school districts to keep teachers from unionizing? (or is the fact that they can get laws passed saying that teachers cannot unionize better than trying to be competitive in the job seeker market?)