IME (and this certainly is what's happened in my career, but I've seen it in many), this is usually a lack of mobility options or undercompensation. Few enough quality workers leave places where there are opportunities to grow and (not or) where yearly raises are in the ballpark of the salary delta from moving to a new position. And if you fall into the trap of working for a bunch of startups, you also get the fun "aaaand sometimes startups just explode from underneath you" effect, too.

For most of the folks I've worked with, it's the case that if they take a job and they're not learning something, they're not going to stay long. And if they take a job and the choices are "stay here, even if they enjoy it" and "take a 15% compensation bump that will ripple through the rest of their career", it would be foolish not to go for the latter.

The places that understand that retaining people takes effort are less rare than they used to be, but they don't grow on trees. If you want to hire someone longer term, I hope you're putting the money, and the investment in that worker, on the table.

That's all fair. At the same time, there tends to be a certain amount of gripe-age with any job and some people are more inclined to try to work through it and others are more inclined to start job-hunting. Which is perfectly fine but, if it's a persistent pattern, I probably won't hire you.

To be clear, I'm talking about a persistent pattern. Even those of us with fairly long-term employment (~10 year stints) in general often have one or two short stints for various reasons. E.g. dot-bomb in my case although the job wasn't a great fit anyway.