Swimming in scuba gear seems almost effortless because of your apparent weightlessness. But water is 800 times denser than air, creating 800 times more resistance to every movement you make. Finning along the reef, you're focused on the bright colors and interesting life, not on the work you're doing. Sweat doesn't run into your eyes to remind you of it. But four hours a day of this is strenuous compared to the sedentary lives most of us lead.
Breathing is a muscular activity that requires energy just like any other.
Breathing dense compressed air, through the resistance of a regulator, increases the workload of every breath. You're not aware of it, you don't feel tired from breathing, but it's a stress you're not used to, and it burns extra energy. Travel itself is fatiguing. It's no surprise that humping your dive gear through airports drains energy. But discomfort, dehydration and anxiety also take a toll.