[_ah]

This may be true, but these vacant roles are generally limited to candidates who have already performed at that level elsewhere.

It's very difficult to gain a promotion while also moving to an equivalent-or-better company. The easiest move is to go up in role while trading down in company, or up in company but down in role. True career advancement happens through internal promotion, or happens if you "trade up" to an equivalent role at a better company.