When the military gets a pay raise, it's across the board - same percentage for all ranks. Because the housing and food allowances are not taxed, those have historically been the places where most of that raise gets put.
Unlike civilian companies, the military has a clear expectation of promotions to higher rank with time - it gets harder and takes longer to get from E-6 to E-7, etc. Which is why the average retiree is an E-7 with 20 years' service.....
I made E7 in 12 years...then spent 10 years stuck there..... The 80's were not good years in the Signal corps for upper level promotions.