Because english isn't as standardized as we would like to think. The written and spoken forms are not a one to one translation. Written english could almost be classify as a seperate language that happens to share words and similar syntax. At least the written version has a modest level of standardization. The spoken language is very fragmented.
I wasn't referring to spoken language. I meant when people try to literally "write like they speak" and end up sounding ridiculous because, as you said, written English could almost be another language.
