What makes you believe that humans can't multitask?
I can move both of my hands at the same time, or rub my belly and pat my head. Both of these examples are me performing 2 functions simultaneously. I can do these because basic motor skills have become a trivial task through my years of development.
Just like muscle memory, which can be highly developed in professional athletes, I think many tasks can be developed to the point of being able to multitask (like when someone says something becomes 2nd nature, or natural reaction).
As others have mentioned, I don't think multitasking is a proficiency in itself, but rather when you become proficient enough at a task your brain can internalize and multitask it.
Humans. Can't. Multitask.
We can switch our attention from task to task in rapid succession and call it multitasking, but that only results in doing many things poorly.
I always giggle at the job descriptions that say they are looking for a "great multitasker at home in a fast paced environment".