Because the language doesn't have the common sense to use an alphabet.因为中文没有按照常识使用字母

To further explain why the Chinese writing system is so hard in this respect, it might be a good idea to spell out (no pun intended) why that of English is so easy. Imagine the kind of task faced by the average Chinese adult who decides to study English. What skills are needed to master the writing system? That's easy: 26 letters. (In upper and lower case, of course, plus script and a few variant forms. And throw in some quote marks, apostrophes, dashes, parentheses, etc. -- all things the Chinese use in their own writing system.) And how are these letters written? From left to right, horizontally, across the page, with spaces to indicate word boundaries. Forgetting for a moment the problem of spelling and actually making words out of these letters, how long does it take this Chinese learner of English to master the various components of the English writing system? Maybe a day or two.

Now consider the American undergraduate who decides to study Chinese. What does it take for this person to master the Chinese writing system? There is nothing that corresponds to an alphabet, though there are recurring components that make up the characters. How many such components are there? Don't ask. As with all such questions about Chinese, the answer is very messy and unsatisfying. It depends on how you define "component" (strokes? radicals?), plus a lot of other tedious details. Suffice it to say, the number is quite large, vastly more than the 26 letters of the Roman alphabet. And how are these components combined to form characters? Well, you name it -- components to the left of other components, to the right of other components, on top of other components, surrounding other components, inside of other components -- almost anything is possible. And in the process of making these spatial accommodations, these components get flattened, stretched, squashed, shortened, and distorted in order to fit in the uniform square space that all characters are supposed to fit into. In other words, the components of Chinese characters are arrayed in two dimensions, rather than in the neat one-dimensional rows of alphabetic writing.
现在再看看另一个决定学习中文的美国大学生。要掌握中文书写系统需要什么呢?完全没有和字母对应的东西,虽然汉字里会重复出现一些构件。这些构件有多少个?别问我。就跟所有关于中文的问题一样,这个问题的答案也是繁复而无迹可寻 ,令人不满。它取决于你如何定义“构件”,以及很多其他冗长的细节问题。这么说吧,有很多个,比26个拉丁字母多多了。那么,这些构件如何组成汉字呢?嘛,你说吧,可以从左到右加到别的构件身上,也可以从右至左,或者从上到下,或者包围起别的构件,或者钻进别的构件里……怎样都有可能。而在这些空间组合过程中,这些构件们或变平,或延伸,或压扁,或缩短,总之会扭曲到能够符合所有汉字应满足的方块区域为止。换句话说,中文汉字的构件们是在二维上排列,而不是字母系统的简单明了的一维。

Okay, so ignoring for the moment the question of elegance, how long does it take a Westerner to learn the Chinese writing system so that when confronted with any new character they at least know how to move the pen around in order to produce a reasonable facsimile of that character? Again, hard to say, but I would estimate that it takes the average learner several months of hard work to get the basics down. Maybe a year or more if they're a klutz who was never very good in art class. Meanwhile, their Chinese counterpart learning English has zoomed ahead to learn cursive script, with time left over to read Moby Dick, or at least Strunk & White.
OK,先不考虑优雅的要求,一个西方人要学中文多久,才能看到一个新字的时候至少知道怎么动笔写出一个差不多的模仿来?难说,不过我估计平均的学习者要花几个月的努力来掌握基本功。要是个从来不擅长图画课的笨手脚的家伙,也许要一年或更多。有这个时间,那个同时学习英文的中国人已经学会了书写英文花体,而且还有空读读Moby Dick,或者至少是Strunk&White。(译者:Moby Dick即《白鲸记》,赫尔曼•梅尔维尔发表于1851年的小说,“被视为美国文学史上最伟大的小说之一”;Strunk&White又名The Elements of Style,即《英文写作指南》,著名的写作指导工具书。)

This is not exactly big news, I know; the alphabet really is a breeze to learn. Chinese people I know who have studied English for a few years can usually write with a handwriting style that is almost indistinguishable from that of the average American. Very few Americans, on the other hand, ever learn to produce a natural calligraphic hand in Chinese that resembles anything but that of an awkward Chinese third-grader. If there were nothing else hard about Chinese, the task of learning to write characters alone would put it in the rogues' gallery of hard-to-learn languages.