I find this observation shocking, because it forces me to confront the idea that the author didn't already have a concise explanation of the book in their head, even during the later stages of finishing and editing the book.
Am I unreasonable to feel that I'm probably not interested in reading a book created by someone who didn't have any overall perspective on what they were doing?
I will say based on my own experience writing a book: ideas you can understand without writing a book either don't warrant a book or probably have a perfectly good book written about them already.
That's why I qualified with "even during the later stages". It seemed to me that OP implied that the book would be completely finalised for publication without the author having gained the required perspective on their ideas. And that would surely defeat the object, since their insight wouldn't make it into the book; otherwise, I agree with your point.
And people read for different reasons, too. You don’t read Wodehouse for the same reasons you’d read Tolstoy or a Missing Manual.
For fiction especially you’ll find that some is meticulously planned out and then “filled in” whereas others are more story idea + characters and see where they go.