One is an example.
Another example would be the employee wants to further their education. Another example is the employee wants to move to another country. Another example is that the employee is at the age of retirement and would prefer to live off of SS/investments. Another example is the employee decides to stay home with their kids. These are all examples.
The point is that when the employee feels that their continued employment is not in their best interests they leave. And yet you seem to have a problem with the employer determining that the continued employment of an employee is not in the best interests of the company. Its a two way street. As long as the arrangement is mutually beneficial it should be continued. Once it is no longer good for EITHER party it should be able to be terminated.