To multitask we have to shift our focus because each task competes within the brain. Talking and writing simultaneously is difficult to do—while they both involve communicating, you use different cognitive processes to execute these tasks. Researchers have actually witnessed the brain struggling with the conflict in MRI scans. They’ve found that even simple tasks can overwhelm the brain when we try to do them at once.
In other words, although people may believe they can multitask, the cannot do so with great efficiency and without minimal stress.