I know that there are younger people who clearly understand they are taking a job as a stepping stone. I know that there are older people who have to transition for various reasons, and they may hope something they accept is a stepping stone.
However, once you reach a certain point in your life, and assuming there isn't some current issue with the economy or whatever that is forcing you to accept something you really don't want, you decide to accept a job with some level of expectation that it will be long term. Ideally any job we take should be one where we would be happy to stay until retirement.
Again, excluding temporary issues like the economy, why would we take a job that we know we won't like?
I think the issue is that companies too often don't recognize how they treat their employees as a whole, and as individuals. A person works at a job, is doing good work and is happy with the environment, and then a new manager comes in that wants to spice things up. Sometimes this results in turnover, and frequently it is the best people who leave.
I know of companies that when times turned bad, they had to adjust things, but they did so in a way that employees felt good about what they did. Most times that isn't the case.
I believe people accept a job believing they will enjoy working there for a long time, and that this changes due to a few major issues - the work environment changes for the negative, the employee decides they want more challenges, pay or whatever, or life events force an employee to make a change.
I think work environments frequently change due to companies making decisions based on what some bad apples are doing (or like one particular tech company specializing in searching the web, some CEO just being stupid!!!), which then affects everyone employed there, and results in turnover among the most qualified.