Because they are two different processes inhalation makes u let air inside your lung and exhalation out of your lungs which is not possible at the same time
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It is not possible to inhale and exhale on the same time. In inhalation we take air and our lungs is filledĀ  different types of gases. But our body need oxygen only so it is necessary to exhale other gases out. So inhalation and exhalation are two different task so it is not possible to do both in the same time.
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not exactly' exhale other gases out'but it also takes water and co2 with some amount of energy .