Well, a lot of traditional unionized blue collar union jobs went overseas. So the employers didn't have a lot of control over that.

A lot of jobs also shifted to smaller companies in areas like tech and employees wanted to take advantage of higher salaries being offered by those firms.

I'm not sure it's a simple case of employers suddenly locking employees in. There were a lot of market forces that made staying at the same single employee forever less attractive from a salary perspective. Sort of a re-enforcing cycle. I'm sure economists have more data.