On a scale of "number of staff who leave" vs "number of staff who threatened with legal action", it really does count as almost never enforced.
Who knows how many employees were discouraged from taking a job with such direct competitors due to this substantial threat?
Just being told by HR that you'd be sued would rationally discourage most people.
It also selectively prevents some of Amazon's best employees from pursuing some of their best career alternatives. Would you like to get into this twisted paradoxical game, in which the better you perform at your job, and the more lucrative your alternative opportunity is (i.e. the more they underpay you), the more likely you are to get sued for taking it?
This is depressing wages and hurting all employees, not just the few chosen for a lawsuit.
Typically, you'll lose your new job, and probably become untouchable by most other employers. You'll be forced to pay a small fortune for lawyers, and spend numerous days in court, with the outcome uncertain, possibly for years, all while you're likely unemployed.
Not a chance most people would like to take.
Imagine you're working on cutting-edge technology, get an offer to lead a team or maybe a whole startup... but you have to wait a year.
For most startups, that's a deal-breaker. They simply can't wait that long.
So that's one key reason California is far better for startups - in other states, they're effectively prevented from poaching employees from big tech.
(Waiting a year is generally fine for big companies.)
Thankfully when it came time to leave Amazon, Oracle retained a lawyer on my behalf to handle those exit negotiations. Made life a lot less hassle.