Over 20 years, you've not demonstrated that growth, and so it makes more sense for a prospective employer to assume that lack of growth will continue. Compare to a junior dev who can bring the same productivity today, but could rise to the level of principal or staff over the long term.
For the record, I would bet that this picture doesn't match reality. My guess is that you've developed knowledge and skills that would make you much more valuable than you might think. But, you are expected to demonstrate them and use them on the job. Good luck!
