"Why are almost all paintings rectangles?" I once asked. Rectangular_Art.

Didn't know then, still don't know for sure now but I bet it is related to the reason almost all books are rectangularly shaped.

Christopher De Hamel reminds that manuscripts were originally written on scrolls of papyrus. We think immediately of the Library of Alexandria. Chinese paintings were also painted on scrolls. De Hamel explains that the advent of a legal system (blame it on the lawyers again) in the West made close reading, where it is necessary to refer back to previous text, imperative and this was easier to do in a codex format than to have to unravel the damned scroll every time. We think immediately of the difficulty we have writing lengthy legal memoranda on our computer where we constantly have to "scroll" up or down to re-read (the undersigned prints his drafts out). Some Western scholar got the idea of using animal skins instead of papyrus. Why were the new parchment manuscripts rectangles? Because the animal was oblong. Just made sense to continue the general shape when writing on it.

De Hamel says by the time of Gutenberg the codex format was accepted practice and they just kept on keeping on.