My personal experience with ex co-workers who have taken other positions is that the main reason for them going elsewhere is because market compensation is highly competitive and most jobs require fewer responsibilities. For one of them, his kid's school location was also a factor: his mother wanted him in a Montessori school and it was a 20 minute drive out of the way every morning.

A simpler way of putting it is this: employers are set on giving employees cost of living increases, but a 3% COL increase can't match the 8% annual increase of the local market. Jobs that were going at 95K a couple years ago are at 115K+ today.