This is deliberate: for many of our readers, their first real introduction to Leanpub (and to other authors' books, bundles, courses, etc.) is on the post-purchase page.
We say at the top that the downloads are available further down the page.
Over time, we will be experimenting a lot with the post-purchase page (including possibly having different versions for existing and new customers), but for now the important thing to keep in mind is that those of us (such as authors, and people who are already familiar with the Leanpub Help Center) who are experienced Leanpub users are the power users of Leanpub, and are not representative of the typical new Leanpub reader.
Our first priority with the post-purchase page is to make it as good as possible for the new Leanpub reader. After that, we will probably enhance it to improve or streamline it for existing readers and authors.