I remember being amused in 1996, that some newbie web developers were writing their html to work best with a specific number of pixels of width... I don't find it so funny these days.
The basic problem is that traditional designers are still trained to think in terms of canvases or formats, which have a fixed width and height. Moving beyond a fixed height is a cognitive hurdle. Moving beyond a fixed width is only more so, especially when bitmaps are involved.