There is a difference with the plugins and themes here, anyone can write a plugin, and upload it with little or no checks being made!
The themes here are the "WordPress Brand" at a quality the public see and expect, there are strict controls by an application and acceptance, where there are suitability rules and code standards.
Example is the Artisteer theme generation product, the themes do not meet the WordPress theme coding standards so they do not get a look in!
They do work with WordPress, this does not mean they are bad or sub standard just not coded the way that WordPress want them to be coded, they also might be missing a few of the requirements.
If WordPress applied the same rules to the plugins we would likely loose 80% of them, so it might be better to create a plugin that generates the child themes.
Hope you don't mind my comments, that is just the way it is, I added a theme, it was accepted on the seventh attempt, and it was the twenty ten theme at the core of it.
Even after acceptance if you change something it has to be re-submitted, and it can fail on something unrelated to the change!