How is written prose more complex than casual speech?

In written prose, it is expected that thoughts are organized and coherent. Unlike colloquial language, we have the ability to edit written language, so it is held to higher standards. Referential words help us make it more concise, if we stay on the same topic for more than one or two lines. Finally, written prose is more likely to reveal hidden meanings on second reading than spoken words that will never be heard again.