If addiction is the chase to fill a spiritual void with physical pleasure, why can’t an addict just follow spiritual pursuits like Torah learning and enjoyment of doing mitzvos?

The answer is that the addict once experienced and chasing the high, is a being of total שלא לשמה , doing not for the sake of G-d, only to fill his own desire of fulfillment. He can learn twenty hours a day of Torah for his own ego, or to fill his own selfish desire to be spiritual. There is no true connection to G-d under such circumstances. All the mussar he will learn has no affect on the deep desire to find the high again.