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Because unless you take half an hour to write 500 words you're always leaving things open to interpretation. A huge part of human communication is interpretation and inference.

When you have a verbal communication people bring up things they don't understand in realtime and you correct as you go so that, by the end of the conversation, a mutual understanding has been reached.

In writing - especially if you're sending emails - people are often reluctant to come back for more info as they think it'll make them look dumb, plus there's the 'inertia' factor plus the factor of not knowing whether you'll get a timely reply.