Exactly, and even if you could do that, where would you set it up? Even the largest bookstore has limited floor space, and that's not even considering those small convenience store spinning racks and the like.
Ebooks are roughly 50-250 KB in size. Figure out how many of those you could fit on a hundred buck TB drive. :-)
Alternately, you must build a rack a mile long to offer the same breadth of selection.