I think if you also navigate to the res folder in the sdk, you won't find ldpi.
Taken from FAQ of Android Holo Colors Generator, but I think it applies to all the asset studio tools
Why the low density (ldpi) is not generated ?
Android Holo Colors Generator is based upon the default holo theme (in Android source code) which does not provide any picture for low density devices. Moreover, ldpi represents less than 2% of the Android Market Share. Finally, if a device is in ldpi, it will use the mdpi pictures.