They earn the privilege of being considered for their service. The quality of their service, the WPPA, the favors they accrue during their service, and political expediency all play a role in whether they advance in grade. It's like any other job: if you have the skills you'll be considered, but it's not a strict meritocracy. Who you know is still important. For example, if you have a bad review in your jacket from somebody who now handles budget disbursements and the other guy is less qualified but has a good review from the same guy, and the division needs new assets, who do you think is gonna get the slot? Will it be the guy who's likely to have the IG scouting around for mistakes, or the guy who's likely to be approved for all new Bradleys?
We nominate generals? I thought they earn that through years of noteworthy service?