Operating systems abstracted the hardware (and other things) rather than having everything embedded in the program (as is typical in many embedded applications

Operating systems abstracted the hardware (and other things) rather than having everything embedded in the program (as is typical in many embedded applications even still today). They don't have to offer time-sharing (aka, a scheduler), but that does constrain them to running a single process to completion before beginning the next. There were OSes before there was time-sharing OSes.