When a computer is "multi-tasking", appearing to work on many things at once, it is really just doing a slice of each task, and then very quickly switching "context" to the next task, doing a quick slice of it, then a quick slice of the next task, and so on, in round robin fashion. Any particular processing core is never actually working on more than one thing at once.

When operating systems are designed, a lot of effort is spent making sure the context-switch between tasks is as fast as possible. If this is not done, significant amounts of time are wasted in the switching, rather than the actual processing of task slices.

While our brains do a tremendous amount of parallel processing (keeping breathing going, heart beats, adjusting hormones, monitoring hearing, all at the same time, etc.) we only have "one" processing core for our thinking. It can get priority interrupts (if a heavy weight suddenly falls on our foot, or some noise startles us) that interrupt our current processing. But generally, switching our attention to a new thing is not very efficient. If you are trying to juggle many entirely different tasks at once, the switching between them is very inefficient. It is more efficient to get full laser zone attention focused one item for a block of time, like 15 to 30 minutes, then go to the next item that needs our attention.