Companies also want to retain the ability to fire at will, with simple HR processes, and to reward based on performance instead of seniority.
Or reward based on bootlicking, office politics and sexual favours. If you think companies are efficient and promote based on merit, I have a casting couch to sell you.
You say that like it's a bad thing. (Also, generally unheard of in the types of places we're discussing; you don't get paid more for being good. You get paid more for be good enough for longer.)