If boss (really boss^n, for any n) makes them miserable, if just make more places relatively attractive. But there are always people quitting from good work places.
Sometimes, an undeniably amazing opportunity falls into the lap of someone who is overall quite happy at their present job, and they quit that job. But that's a fairly rare case.
Is that true? I don't know of any studies but I give a new company like a year before I start looking for something better. Now I've been in the same position for 6 years but I've been looking for something "better" the entire time and actually turned in my notice twice but ended up staying. No offense to any employer but they have never shown loyalty to me. I'm not giving it to them.