Mostly because, unless you're in a major metropolitan area (and even sometimes then), they don't pay squat to anyone but the few big anchors. And just like everyone else, these folks would like to get paid more money.
So they start out somewhere, work a few years, get some experience, then apply for jobs elsewhere, where they'll get more money and more responsibility. This will go on for a while, until they're happy with the pay, or the location, or both.
I live in a small city of ~100,000, and the evening news anchors have been around for a while, but the junior reporters come and go. The ones that really like this area will stick around, making diddly, until they get enough seniority that they're not junior reporters any more.