McDonalds gets ALL of it's managers from inside, those that see that serving at the counter is not the end all and aspire to do more.
Macdo's rewards those people with more money.
You hit on the key concept, ambition.
It stands to reason that as long as people have different ambitions, they will have different standards of living.
The point of socialism is to crush peoples’ ambitions, and make them accept whatever the State allows them to do.