Q-DOS from which MS-DOS is derived is basically a clone of CP/M. I've written a CP/M bios for fun to bootstrap my IMSAI. It's not really that high level. You got even more from IBM's BIOS but of course that was too slow for what people wanted to do with those machines - so everyone talked to the hardware directly.

In those days it didn't matter what you were doing - the hardware was too slow. It's not like today where you can trade off performance for reliability or cleanliness. People praise Wozniak for his genius of trading hardware for software, but the tight coupling made it really hard to make future iterations. People didn't value upwards compatibility yet.