[mitosis]

Chinese writing does not encode sounds. It encodes meaning. If you write Chinese in pinyin, all you do is write down the sounds, not the meaning.

Said differently: why do we keep writing English like this, instead of using a phonetic alphabet? Same reason.

[prewett]

Some of the characters do actually encode sounds, typically in the right side of the character. However, the encoding is more like a memory aid than it is phonetical. 中 (zhong1), 钟 (zhong1), 种 (zhong3/zhong4); 艮 (gen3), 跟 (gen1) 根 (gen1), but 很 (hen3). And it doesn't always work: 立 (li4), 位 (wei4), 拉 (la1).