In some cases it is hidden though. USB has a lot of "generic" device classes where (at least in theory) the OS only needs to deal with a standardized interface and whatever actual hardware is behind it is driven by an embedded controller.

It's just that most internal hardware initially only cares only about Windows (or only Linux) so it makes more financial sense to develop a more complex driver than a complex firmware. The equation might change later on but by then you are stuck with the hardware design.