That’s about as logical and as loaded as an assumption as being fickle. It could also mean the person isn’t qualified and other employers figured that out. But again these are assumptions. In their shoes they are right to be wary and probably have some experiences backing up that caution.

For junior positions maybe. For senior and especially principals there is a ton of value to continuity. When a senior engineer leaves it's almost like replacing the entire team in terms of overhead if there isn't a natural successor. And when principals leave you end up losing vision as well as that leadership. This can kill entire projects of it happens unexpectedly.

My position required at least a year to learn everything, and I'm a pretty fast learner. My coworkers jobs require a similar level of training, even with experience. If a candidate spent less than 2 years at their 3 most recent jobs then I agree with my manager that they weren't worth potentially wasting time on.