There are a number of reasons
I do not believe hierarchy should be expressed in urls in a CMS because you should be free to move pages around and change the hierarchy without changing the urls because cool urls don't change. Folder hierarchies are a limitation that originated in sites using static html files organized into folders, it is not a limitation we should artificially impose on a CMS where we are not really serving files from a disk.
I do not believe there is any SEO benefit to having extra folder segments in an url. Shorter urls are better and if you really feel the need to pack an extra keyword into the url you can use a - instead of /
The Url re-writing logic is very complicated already in order to support running as a root level site, running as virtual directory in a sub folder, and running multiple sites based on a first level folder name in the url for child sites. This was all difficult to implement and would be astronomically more complicated if we had to account for extra arbitrary segments in the url.
In short, in the cost benefit analysis, I see the cost as high and the benefit as nearly null.