Well, they had incentives. I came out of Basic as an E-3 because of college. I got an early education in what the military would be like, so declined 2 or 3 chances to go before a below-the-zone board for early promotion, survived 2 or 3 suspended busts and got out a Sergeant.
That was something. Because of high school ROTC, I went into USAF boot camp as an E-3. Program had to be new as the TI suggested I put Airman Basic ( E-1 ) on everything that wasn’t official just to keep things uniform. Later I found out at my first base that there were airmen who went 4 years and made E-3 before they got out. That was normal. Sure was fun in tech school as I outranked most of my classmates. After I was in a few years, E-2s and E-3s coming out of tech school were fairly common.