I have developed a theory about this:

Suppose you were hired to do X, and after doing X for a few years, you have developed the skill to do Y instead. But that skill does not guarantee that your current employer will have an opening for a job doing Y. Even if they do have the opening, from their point of view, if they let you transfer, they will definitely need to hire a replacement X, and you might turn out to suck at Y. So a rational manager will feel a strong pressure to leave well enough alone and let you keep doing X.

There are a variety of ways for an employee to escape this situation, but the simplest one is just to find another company with an opening for Y.