In my experience it really isn't that rare. There are a lot of people who do not job-hop nearly enough to optimize income (maybe they're optimizing for stability though, that's their business) and once they've made one jump there's such a possibility/probability of doing so again that it's not really in an employer's best interests to make it easier for them to do so by taking advantage.

Most engineers only reach their steady-state productivity 9-12 months into a new job. Getting a year of somebody cheap is rarely better than getting multiple years of them unless you never should have hired them at all.