Ichigo didn't either, remember? It's because they leak so much spirit energy that it forces their zanpakuto to enter shikai permanently.
And in the anime and manga, Kenpachi says that he thinks his sword is like Ichigo's. Then later on, Yoruichi tells Ichigo that Kenpachi is probably like Ichigo in that his sword is stuck in shikai.
Then, in the character book, it explicitly says that he has permanent shikai so there can be no doubt about it.