Because if you tell it to print a 4x6, 5x7, or 8x10 to print on an US letter, it will center it on the page. If you ask Word to print a document with 3" margins on a US Letter paper, it will fit it onto the center of the 8.5 x 11 inch paper. Same idea, but with LESS control.
With iPhoto, you are asking it to do the same thing. "Here, iPhoto, print this small picture on this big piece of paper." It's only option is to print it in the center of the paper. If you want to print a 4x6 print on 4x6 inches of paper, you have to tell it you are using 4x6 paper. If you don't have that, then you are not going to get the results you want with iPhoto. Not designed that way. Feedback to Apple via the Apple website or the application menu (if it is there) is the only way you can ask for that capability. Third party apps like ImageBuddy can lay out the photo whereever you want it on the page.
Jim