David Meyer probably believes that

Choices:
A. Multitasking is hard
B. not enough information
C. It is better to multitask
D. People are too busy
Multitasking is hard
In spite of the fact that people brag about being able to multi-task, the reality is that the human mind can only focus on one thing at a time. When you pay attention to one thing the mind automatically blocks out all other stimuli. Multi-tasking is simply jumping from focal point to focal point; but, you are still only focusing on one thing at a time.
The payoff from trying to multi-task is not worth the toll it takes on the individual making the effort.
David Meyer, PhD, a professor of psychology at the University of Michigan states the following:
Doing several tasks simultaneously may seem like the height of efficiency – and it would be, if a person had more than one brain. In the real world, multitasking actually wastes time and reduces work quality.
Many studies have shown that multitasking slows down the mind and makes it less effective. Dividing and distracting the mind causes rapid mental course changes, the inability to effectively focus, and creates stress – which increases with every additional task that is added to the mix.
Imagine a dog who is trying to chase three different balls thrown in different directions. He runs two steps in one direction, three steps in another, and one step in the third. He ends up running in circles.
The mind is the same.
The more time you spend changing mental direction the less time you have to actually focus on a single task.
The more directions or mental tasks you add, the less effective you become.
But . . .  that is not the whole story. Another facet of multi-tasking is worrying about events, projects, or situations that have nothing to do with where your focus should be at the moment. Worrying about what has happened, or may happen changes nothing. All it does is drain your mental and emotional resources, making you less effective at the task at hand.
Let your subconscious handle it. The subconscious mind is one of the most powerful tools that we possess. It is unnecessary to make an effort to keep everything at the “top-of-mind” because everything that happens is stored in your sub-conscious.