Yeah, this is my experience too, trading up every couple of years (though sometimes I've taken a sideways or slight downwards step to broaden my experience or work for somewhere that I thought was interesting or worthwhile). I've had a couple of longish spells at companies, 7 or 8 years, and in both those cases I left because my pay wasn't keeping pace with the market or my growing responsibilities and management wouldn't budge. The irony being that at those places i was at my most valuable because of my institutional and political knowledge. I think companies greatly undervalue the benefits of long term employees.

Same here. You don't even know what you're worth if you keep working at the same company. And even then, if you don't know what you could ask, you will never know how much you're really worth.

Let's say you could command 100k/year but your first job is 30k/year and stay there for several years because you like the work environment. Then you'd go to the next company and ask for, I dunno, maybe 50k/year and think that's a huge bump. If you saw salary ranges for similar positions elsewhere you'd be more inclined to start looking around even if you were happy at your current employer.

Nowadays things are a bit more transparent in the tech world with stuff like levels.fyi (but you have to know about those), but I'm thinking it would benefit people in all fields.

Just taking what you're offered is understandable when you're young and don't know any better. I think a lot if not most people had that experience with their first job(s). But there's no excuse for not taking agency over your own life when you're a bit older and more experienced. These people have to grow up and stop acting like kids, or accept that people will take advantage of them.