Not everyone can afford to just walk away more than once or twice.
And people may perceive the uncertain alternative of not getting that job right now as much worse than it would actually turn out, and agree to stuff they don't really want. Like the point made in this short comedy scene: https://www.youtube.com/watch?v=-yUafzOXHPE
What's making these people agree to the non-compete? What's stopping them from just walking away and not signing?