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You could take the question a step further and ask: Why isn't anyone following Imams Awza'i, Sufyan Ath-Thauri, Tabari etc? Because either they, or their students did not completely codify their madh-hab, thus causing it to eventually die out. A person can't practice complete deen with an incomplete source of masa'il. I've seen in aqeedah and fiqh books that when their positions are in accordance to the 4 madhaahib, then it's accepted, otherwise not.