Because you can't cheat by using cognates.因为你不能取巧使用同根词。

I remember when I had been studying Chinese very hard for about three years, I had an interesting experience. One day I happened to find a Spanish-language newspaper sitting on a seat next to me. I picked it up out of curiosity. "Hmm," I thought to myself. "I've never studied Spanish in my life. I wonder how much of this I can understand." At random I picked a short article about an airplane crash and started to read. I found I could basically glean, with some guesswork, most of the information from the article. The crash took place near Los Angeles. 186 people were killed. There were no survivors. The plane crashed just one minute after take-off. There was nothing on the flight recorder to indicate a critical situation, and the tower was unaware of any emergency. The plane had just been serviced three days before and no mechanical problems had been found. And so on. After finishing the article I had a sudden discouraging realization: Having never studied a day of Spanish, I could read a Spanish newspaper more easily than I could a Chinese newspaper after more than three years of studying Chinese.
我还记着，当我刻苦学习了中文三年的时候，有过一次有趣的经历。有天我正好在旁边座位上找到一张西班牙文的报纸。我好奇地拿起来看，“嗯～”我想说，“我从来没学过西班牙语。看看我到底能懂多少。”我随机挑了一篇关于空难的小文开始看。结果我发现稍微猜一下就能获取大部分的文章信息。空难发生在洛杉矶附近，186人遇难。没有幸存者。飞机起飞后一分钟后即坠毁。飞行记录上没有什么特殊状况的提示，而塔台则并不知道任何紧急情况。飞机三天前刚维护过，也没发现什么机械故障。等等等等。看完文章后我突然沮丧地意识到：从没学过一天西班牙文，我读起它的报纸却比学了三年的中文报纸还容易……

What was going on here? Why was this "foreign" language so transparent? The reason was obvious: cognates -- those helpful words that are just English words with a little foreign make-up.9 I could read the article because most of the operative words were basically English: aeropuerto, problema mechanico, un minuto, situacion critica, emergencia, etc. Recognizing these words as just English words in disguise is about as difficult as noticing that Superman is really Clark Kent without his glasses. That these quasi-English words are easier to learn than Chinese characters (which might as well be quasi-Martian) goes without saying.
这到底是怎么回事？为啥西班牙这个“外语”这么容易？原因很明显：同根词。这些同根词跟英文词汇相比只有小小的改造。我能读懂文章，因为绝大多数关键词基本都是英文：aeropuerto, problema mechanico, un minuto, situacion critica, emergencia,等等。认出这些词儿不过是一些英文词穿了马甲，这难度大约和发现超人不过是肯克拉克不戴眼镜的难度差不多。不用说，这些类英文词比中文汉字好学（中文汉字则多半是类火星文……）。

Imagine you are a diabetic, and you find yourself in Spain about to go into insulin shock. You can rush into a doctor's office, and, with a minimum of Spanish and a couple of pieces of guesswork ("diabetes" is just "diabetes" and "insulin" is "insulina", it turns out), you're saved. In China you'd be a goner for sure, unless you happen to have a dictionary with you, and even then you would probably pass out while frantically looking for the first character in the word for insulin. Which brings me to the next reason why Chinese is so hard.
想象一下，一个糖尿病人在西班牙发现自己需要注射胰岛素。他跑进诊所，只需很少的西班牙语和猜测的过程，他就能获救（其实，英语"diabetes" 翻成西班牙语就是 "diabetes" ， "insulin" 等于"insulina"。）在中国呢，他肯定完蛋了。除非他带了一本中文字典，即便如此，他多半也会在字典里疯狂地查胰岛素第一个汉字时不支晕倒。这正好说明了我下一个要说的中文难的原因。