Writing is a skill and, like other skills, has to be learnt.
In fact, writing is a lot more than just using accurate grammar and choosing appropriate vocabulary. Writing accurately is, of course, important, but there are a lot of other aspects to consider. Every language has special phrases which can be used for a particular purpose. For example, if you are writing a letter in English, it is usual to start by referring to a previous letter or meeting. If you wish to offer the reader further assistance, there are special phrases you can use at the end of your letter.