Because the folks staying at a single company for decades or just moving around various "Programmer 1" and "Programmer 2" jobs are not good developers. In my experience socially and professionally, the best developers are very quick to move to another position at another firm. Whether it's for the money, the better title, fewer working hours in the week, or whatever I'm not sure is the point, but that they're not sticking around waiting for their current circumstances to change.
And most companies only have 3-4 levels of developer before you start leading teams, mentoring, etc. Even those that specifically have architecture tracks where you can do more senior-level technical work without any management or teaching overhead will only have a few of those positions as well.
Stagnation is death, and maintaining the same position for a decade is a pretty good definition of stagnation.
Why do the current circumstances have to change? What's bad about having a job that you actually have experience with.
That's like saying that you should divorce every few years because otherwise you seem like a bad husband... Rich people and prominents do it all the time, after all.
It's sad tho', interviewing is a pain in the backside, I would love to stay with and grow with an organisation for a solid chunk of time, 10+ years. But it seems impossible - far easier to get a promotion/raise/transfer whatever by moving. And this makes no sense for companies either!
Why can't you use your experience to actually be a GOOD developper? Why do you have to manage or consult something?
Would you say the same thing to e.g. a surgeon?