[viraptor]

Or a more mundane reason: You need to add the dimensions to the image to reserve the page side and prevent skipping. That takes extra effort and is not something people think about. If there are any places that a/b tested changing layouts specifically, I expect it's extremely few. It's much easier to make the background a clickable ad area and some pages do that explicitly.

[kwhitefoot]

Why does anyone need to add the dimensions? Surely the build process can do this ahead of time or the server can do it on the fly. It only needs to be thought of once.

[viraptor]

You still need to either use a fully integrated dev/build system which allows it, or explicitly add the placeholders. The build process of course could add the actual numbers, but I don't think I've ever seen that happen outside of CMS content. (Specifically content, not even the theme / static stuff)