> "zh" is actually one of the more reasonable pinyin digraphs because it follows the same pattern as "sh". If "s" + "h" results in [ʃ], then logically "z" + "h" should result in [ʒ].

Note that the sound [ʒ] is common in Mandarin, but its pinyin spelling is "r". "zh" isn't voiced and is affricated.