User blog comment:WillofOP/On another level!/@comment-2236761-20111013205231/@comment-1362207-20111013210121

Because there is no evidence that supports it. Haki was explained specifically by Rayleigh and said nothing about it being able to make you grow three heads and six arms, or enveloping your leg in flames. So until its stated in the manga or stated by Oda himself, Zoro and Sanji DO NOT utilize Haki.