Mostly because the os needs to coordinate everything and not all operations can be interrupted. Operating system maintain each program illusion to be alone on t

Mostly because the os needs to coordinate everything and not all operations can be interrupted. Operating system maintain each program illusion to be alone on the hardware, virtualizing disk transfers, memory mappings, display regions and all the other hardware. Some operations once queued and dispatched need to complete before other operations can be accepted - that might happen for example when the os is figthing to access a memory page from disk but the disk is busy in a large dma transfer - then the illusion of being alone comes crashing down and no clever scheduling can fix it, because a class of operations without a guaranteed time constraint exist on the ibm pc platform. Without a hard real time guarantee from the hardware, everything built on it cannot provide a guarantee on dispatching only the exact amount of operations that the system can handle. And that happens even before accounting that the os or some driver within may have some cpu locking going on.