[Tycho]

To me it's pretty obvious and it's because of a simple, logical dynamic. If you apply for a better role within your firm, your current boss works against you. They may either block your move completely, or delay it. If you apply for a better role in another firm and get an offer, there's no way for your current employers to act against your interest.

Companies can foster better internal mobility but they can never change this basic dynamic.

[gaius]

Dead men's shoes, this is called. You can only get promoted if the guy above you, for whatever reason, goes away.