Mrmikel's initial reply is correct. To rephrase the succinct points:
- Central to EPUB is flowability; the ability of the reader to break lines to fit the text to different screen and text sizes.
- Central to poetry is (near) absolute control over placement of text.
Two diametrically opposing aims...
EPUB3 does allow fixed layout (i.e. absolute control), but it is not what the format is intended for. For that we have PDF.