People wind up _staying longer than seems rational_ because they develop habits (schedule, commute, favorite lunch spot, etc.), develop relationships with their colleagues, feel a sense of loyalty to the company/their superiors/their team/etc. (this is often misplaced, but irrationality rears its head yet again), don't want to go through the hassle of interviewing, don't want to have to change doctors if/when their insurer changes, etc., etc.
Precisely. Rational? It's a multivariate problem. Get married, buy a house, have kids. Now you're juggling daycare and school schedules and dependent benefits. At the same time, you have to balance developing enough technical expertise to increase your value, learning new tech to say current (while likely having less time for learning), and paying close attention to your work projects to be sure you can reduce your accomplishments to a cold, hard ROI so the angel of death passes by when the economy slows and the RIF times come.
And while more junior engineers can be forgiven 3 jobs in 5 years as they ratchet themselves up, seniors need to demonstrate they can stay somewhere for long enough to amortize their hiring, while avoiding getting stuck in a rut.
Again, this doesn't match my experience. True, I am a contractor instead of an employee, but I generally tell everyone beforehand that I'm mainly interested in 3-to-6 months contracts, not long-term. I've never had any problems finding contracts.
I'm not unique: I have a friend (also a contractor) who managed a single 6-month job in the last ten years. Everything else was shorter.
People wind up _staying longer than seems rational_ because they develop habits (schedule, commute, favorite lunch spot, etc.), develop relationships with their colleagues, feel a sense of loyalty to the company/their superiors/their team/etc. (this is often misplaced, but irrationality rears its head yet again), don't want to go through the hassle of interviewing, don't want to have to change doctors if/when their insurer changes, etc., etc.