Well, one reason is that if you put the text into the icon (as in part of the image) then you have to have a different set for each language. This can really become difficult to maintain, as well as bloat the package.
The other reason is that it hasn't really been discussed. I wouldn't mind seeing it set as a user-option of whether there are text-labels under the image or not. THis way, each user can control it; and it can be incorporated into the language files which makes it really easy to update.
If you're good with PHP, you may make a try at modding the current RC SVN build and write to the dev-list about your mod and what it does.