I've always had a feeling that it's to differentiate iPad from iPhone. If they had the exact same apps, right down to a scaled up Calculator with gigantic buttons on the 10" display, it might make it less likely for consumers to think of iPad as a separate concept from iPhone. Calculator and Weather wouldn't really translate that well to a huge screen, anyways.

And, of course, there are several great third-party apps to fill the void if you need them.