Books of musical notation are, so far as I can tell, the only books whose pages need to be turned in an expedient, rhythmical fashion. If you're sitting at a piano and trying desperately not to think about what it is you're playing and you weren't fortunate enough to grab someone to turn your pages for you, the last thing you need to worry about is that the page turn you're coming to is in the middle of a particularly difficult passage.
Rather than trying to grow a third arm from your abdomen, spontaneously developing your telekenetic powers or trying to flip the pages back with a sudden movement of your head, sheet music publishers have been thoughtful enough to space their staves in such a way as to place the page breaks in the most unobtrusive locations possible. The most extreme example of this is that occasionally a page needs to be left blank in order to make the music playable in a fluid manner. "This Page Intentionally Left Blank" is printed on the skipped pages so as not to cause a trainwreck.