Why do people write poorly?
The main reason people write like they talk is because they don't do enough reading. When you are doing a lot of television watching, it makes you lazy. Reading on the other hand, stimulates the brain. It forces you to do reading comprehension as you read. If you are poor at reading, you will more than likely struggle with writing as well. Fortunately, the easy fix is to do more reading that will either challenge you or stimulate you to read more.
Is the subject of Literature just reading novels, poems, and prose?
That is just the tip of the iceberg actually. Literature involves understanding the history around the time of that particular piece of work. After you have done that, it is easier to understand the jargon and interpret the meaning of the work and perspective of the writer.
Why is it important to master the English language?
It is important to master the English language so you can articulate your ideas in written and oral form. It is very evident that you have not mastered it if you have misspelling, poor punctuation, and grammatical errors in you communication. Mastering English is a skill that everyone needs to embrace.