Simple. It only runs one what you think of as a "program". I've written for bare metal on several platforms. Operating systems exist for a specific use case:

Simple. It only runs one what you think of as a "program". I've written for bare metal on several platforms. Operating systems exist for a specific use case: when you want to run potentially multiple programs on the same piece of hardware, over time (not nec. concurrently), and you don't want your software to have to think about how to interface with the hardware. That's what operating systems do. There are plenty of cases when you don't need, or want, an OS.