The olfactory system is the most ancient sensory system, and inhalation is necessary to bring airborne chemical information into the brain.

“So before vision, before audition, before touch, before all of that, the olfactory system is the most ancient system. And the olfactory system, of course, is designed to detect chemicals in the environment. And so if you imagine an early organism that perhaps we evolved from or perhaps we didn't but nonetheless that we share some features of, at least in terms of olfactory function, in order to get that chemical information into the brain, you need to inhale. You need to bring that information in.”