Sites are different in their look and feel.
Some sites have a large content area. Others put more weight on the widgets part of their sites.
There are sites that use unique style and sites that focus on the topic they want to deliver.
But above all, webmasters want to master and control the ads on their sites in any way possible.
As known, premium publishers have this kind of feature where they can customize not only the size of an ad but also any element of it - color, font etc etc.
The question in asked - Why Adsense wouldn't give (or leverage) this ability to all (or mid-size) publishers.