Why is reloading of sources not proposed?

After starting Ubuntu Software Center (USC) after a fresh install of karmic, a lot of software was available to browse, with their descriptions and all; however most of them were non-installable. No error messages were shown, just the "File > Install" menu was disabled, and no "install" button was made available in the software description. I was puzzled. Fired up synaptic, reloaded my sources, and then much later I retried USC, and noticed the software were very much installable.

To a novice user this state of affairs may be quite puzzling, hence my question, why no messages were shown or the user prompted to reload the sources without having to load Synaptic (most novices are unaware of it)? Or am I missing something?

Thanks