The why has already been provided in this thread more than once. People don't want to go back to work to the places that laid them off, or were underpaying them, or were just shitty places to work, when they can get better jobs elsewhere. I don't know why this is so hard for some people to understand, or why they insist on blaming the workers despite all the evidence.
Anecdotally, I learned this lesson a long time ago. I was a top-producing manager who had developed a kickass team, and found myself making less than my reports. So I went to my management to re-negotiate, and they told me that to do that I would have to leave and then come back. So I left, got a better job elsewhere, and didn't look back.
I applaud anyone else with the courage to do the same.