Since we live in an instantly changing world, both the employee and the employer have been varying in standards and desire for each other. Therefore, the two factors work together resulting in the view that no one could stay in one position permanently for the whole life.
In one hand, once people get into a certain environment which triggers them to adapt to it as well as reform properly, people’s desire for better condition could be formed. And it is the time that the current company could virtually not afford their higher demands ranging from the welfare and their status in the firm. I suppose that the newly graduate students are appropriate in the case. As far as I know, the lack of effectively and clearly recognizing their own limited capability could finally make the students hop job frequently. Also after they settle down and with their real ability in career rose, they would start a new search for company that could meet their needs. It is just like the process where a man eventually built his family after many women were through.
In the other hand, company itself could shift a lot during a period which lead its progress or bankrupt. And in that case, the company could relentlessly abandon the employees who are not able to catch up its progressing path or release the employees for the sake of its failure in the business. Especially the Depression years, when many people lost their jobs and could not afford their living, made them to be back to college for new skills for the future career. In short, every reformation of a company could create a number of employees laid off.
Anyway, the claim that one could work for only one firm for his or her life seems to be ridiculous to me, since the reality of the society could not make this dream realized.