>Why companies dont value their current employees is beyond me.

It's because they take advantage of their current employees and get away with it. They could promote you regularly, or they could take the gamble that they'd save money and you'd stay put rather than uproot yourself and spend effort job searching (which is like taking on an additional part time job itself), and if you did uproot they'd just offer the same salary to someone else. They don't value institutional knowledge because they feel they've compartmentalized the jobs enough. Going from the mailroom to the C level sounds a lot more like a bad movie rather than something even remotely possible today.

this is a known short cut in consulting. Quit at your current firm, goto a competitor then come back 2 years later at a level up. Had you stayed at your current firm it may be 3-5 years to level up.