Why? Because it's a falacy! It's bubkis, heresy, artifice, flawed, a delusion of the human mind. At best, a person is performing rapid task switching and typically performing all the tasks poorly! If you doubt me, check out this video from CNBC, and feel free to comment on your results:
Travis indicates a few studies I wasn't aware of, but the truth of the tests was the same. People are slower when performing "multitasking" and the likelihood of making a mistake are greater.
So why do we do it? Because Busyness is a sign of Success to more people! We equate busyness with getting things done, but I have to ask: Are we doing the right things?
Honestly, how are you addressing your day? I am trying to make better use of my time when I can. What I am trying to do, typically, is schedule my day to get the primary activities done. I also schedule time to look at emails and return phone calls. This way I am not feeling the need to check on some of the biggest distractors of my day. If I finish a task early, I will either move on to the next, take a break, or return some missed calls.
In doing this, I am finding that I accomplish more throughout the day, and less seems to be dropped. Let me know what strategies you have to get your work done. I'd love to learn how other people are effective.