I think a lot of businesses anticipate a higher-than-normal voluntary turnover rate, so they aren't going to go out of their way to retain employees who turn in their notices. I remember working for an insurance company as a temporary worker, and my supervisor said he knows there are three or four other insurance companies in town that can offer higher wages and better sales leads. If someone he hired really wanted to resign voluntarily, there really wasn't much he could do to discourage him or her.
I feel sorry for employers who have to lose loyal employees because of circumstances beyond anyone's control. Turnover in general can seriously affect productivity, because another employee has to stop working on other projects to train the new hire, and it takes time for a replacement to get up to the speed of the former employee.