After they found Kalani where they did, I doubt anyone would actually question what happened to his other friends. He also told them he came from a place that got wrecked by zombies so they probably all assumed that he was alone now. He also (if I'm remembering correctly) put it in the past tense. "There was three of us." So he implied that they were dead, and he was never pushed further on it.
As to why he did it.....your guess is as good as mine.
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