You are hardly alone in your experience of finding it easier to speak fluently in English than to write well in English. The experience is actually universal, and we can be sure that it’s true not only to learners of English but of most—if not all—languages as well. This happens because of the vast but not well-appreciated difference between spoken language and written language, and in answer to your question above, I’ll explain the reason for that difference as simply as I can but hopefully without being too simplistic about it.
IMAGE CREDIT: CNBLOGS.COMTHEY ASK: WHY IS IT TOUGHER TO WRITE THAN SPEAK IT?
When we speak, we simply repeat familiar phonetic sounds to convey ideas that have been clearly imprinted in our minds over time by just listening to those who speak the language, in much the same way that young birds learn bird language from the sounds made by their parents and the rest of the flock. The communication medium for speaking is the sound itself and our message gets instantly validated by our own ears and also by listeners other than ourselves, if any. The communication loop is therefore short and joined almost instantaneously.
In contrast, when we write, communication becomes a much slower, highly abstract, and complicated process. We need to harness the many tools of written language—vocabulary, spelling, sentence construction, punctuation, grammar, structure, paragraphing, orthography, typography—and map them on a physical surface (paper, board, or computer screen) to make sense and to clearly convey our ideas not only to ourselves but more so to the unseen or unknown reader. I’d say that just to learn to write passably well in a particular language is a no mean feat, but that to write so well as to become a great writer in English like William Shakespeare—a native English speaker—or Joseph Conrad—a nonnative English speaker from Poland—is nothing less than a stupendous achievement.
So I’d say that there’s no reason for you to fret that it’s not a breeze making yourself as fluent in your written English as in your spoken English. Mastery of written English is a long and continuing process, but from your postings, I can see that your written English is already way above par. I have no doubt that through sustained reading of excellent books in English and more practice in writing in English, you’ll eventually become as fluent or more fluent in your written English as you are now in you spoken English. Then perhaps—who knows?—you’ll begin to see your byline in leading English-language periodicals or in your own nonfiction books or novels, in much the same way that Michael Crichton did after getting his medical degree from Harvard.
Wouldn’t that be great?