Health insurance was probably a factor, but in general if your employer offers insurance they have to offer it to everyone in the company and the insurer can’t deny them. But until COBRA, there was no option to continue coverage after leaving, so if you needed insurance you basically had to make a jump.

I think it’s much more likely that it’s a combination of a few things:

1. there used to be real rewards for sticking with a company for decades in terms of pension and seniority. If you got a pension worth a third of your annual salary for the rest of your life after 20-30 years and you otherwise liked the work okay, there’s a real inducement to stick it out. Pensions are mostly a thing of the past now.

2. It was harder to hire (no LinkedIn or Indeed) and people didn’t move around as much, so promoting from within was more of a thing as applicant pools were smaller and more local.

3. the idea that you should stay somewhere for decades was ingrained in the boomer generation because it worked for them. Gen X not quite so much but it was a much smaller number of workers. And it’s kind of expected to jump around a lot in your 20s/early 30s while you’re finding a career. We’re only getting now to a point where the millennials are hitting 40 and are STILL jumping around frequently, because now optimizing long term financial security incentivizes maximizing salary and seniority over duration.