Which many people would be happy to do. Employers are the ones that made the first moves on breaking loyalty, not employees. The bulk of employees are not looking to make moves if they feel they are being treated fairly and compensated well.
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.
Who exactly was sending these blue collar jobs overseas if it was not the employers?