The premise that a book can be summarized into a few key take-aways seems reasonable - non-fiction authors are required to summarize in that way as part of the contracting process, since it demonstrates the book has a clear set of arguments or aims. However, a primary function of the other sentences is to offer evidence that supports the central propositions. One could summate The Selfish Gene in a few sentences, but why should anyone be persuaded by those take-away claims without any evidence? We don't need to take away the bulk of the evidence in detail (the majority of sentences), but we do need the evidence in order to have faith in the key ideas.
The premise that a book can be summarized into a few key take-aways seems reasonable - non-fiction authors are required to summarize in that way as part of the contracting process, since it demonstrates the book has a clear set of arguments or aims. However, a primary function of the other sentences is to offer evidence that supports the central propositions. One could summate The Selfish Gene in a few sentences, but why should anyone be persuaded by those take-away claims without any evidence? We don't need to take away the bulk of the evidence in detail (the majority of sentences), but we do need the evidence in order to have faith in the key ideas.