Because biologically they're suited to preforming household tasks which require less consistent focus and watchin over the lil'uns, campfire. Multitasking.
Also I read somewhere in some scientific journal multitasking also decreases your ability to focus on a big problem and solve it with big brain thinks. Makes you dumber, destroys your attention span, makes it harder for you to do something like write a book or train in a martial art.
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My experience with multitasking is that I have memory issues and I end up forgetting to take my meds and bathe, so, like I highly don't recommend that. Just focus on one character at once, you will develop your mentality and patience, and be a calmer, more centered person, with a better sense of grounding in what's going on in the world.