Speaker is unprepared to connect with the audience or respond to its feedback. Delivering the speech without notes is difficult because there are so many details to remember. Why speech is different than writing Speech and writing involve different parts of the brain according to medical studies of people with aphasia or brain injuries. Speech pre-dates writing in human evolution. Children learn to speak without schooling but require school to learn to write.
People can communicate with domestic animals using body and vocal language.
Speech is structured as a two-way dialog between the speaker and audience. New words and phrases are added to the vocabulary of speech faster. The audience and the situation are known in speech. The writer and the audience are separated in writing. When a speaker writes a speech, the thoughts are designed along literary lines into formal sentences and paragraphs using literary parts of speech and constructions.