Switching jobs can be VERY disruptive, especially if you’ve worked somewhere more than a year or two. I switched jobs about 3 years ago (3 years ago this coming Sunday in fact), and it was a colossal change coming from a job where I’d worked for nearly 10 years prior. At the old job, I was well tied into the gossip, knew who had the titles, who had the actual power, and who did the actual work. I also knew what/who I could and couldn’t blow off. In a less work-focused sense, I knew where to get lunch, where to run errands, how long those things took, the best ways to and from work, etc… Changing jobs meant nearly all that work and near-work knowledge went out the window. I had to relearn all that stuff in my new job, and I’m still learning (it’s a much larger enterprise!).
I think that a lot of mid-career people are busy enough with families, aging parents, and other stuff such that the disruption for a new job isn’t worth it unless things just go to complete shit, or they get an offer they can’t refuse. More often than not though, they act like the proverbial boiling frogs and don’t realize just how shitty things have become, because it happened over the course of many years.