Do you mean, why do speakers speak differently than they way they write? If so, the simple answer is, Time. Speakers can speak much faster than they can write, so they combine sounds, omit sounds, even words and phrases. Also, when you're talking to someone, you have the opportunity to ask them to repeat what they've said and/or to explain what they've said, whereas with writing, you don't have the chance to ask the author questions, so the author has to leave in all the sounds, all the words, and all the phrases, so that readers will be able to understand what's being communicated.
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