The reason it isn't, is because it's automatic. Your brain keeps you breathing as much as it can (if you hold your breath until you pass out, your brain will start breathing again for you). Breathing isn't reward driven. It doesn't engage the dopamine system the same way, eg cocaine does. You don't become tolerant to breathing the same way you do, eg cocaine. Lastly, for something to qualify for Substance Use Disorder (SUD), they need to keep doing it, despite social and health ramifications of continued use in the face of developing a tolerance for it. Other than some edgelord shit, no one's gonna give you shit for continuing to breath.