The problem with this idea is that many who install such programs never perform an update again. Thus, if you included the detections with the main package, they'd be using the detections from whenever that package was released, which is obviously not good.
Additionally, the main package of Spybot is generally released only once every year or two for several reasons. First, the package itself and the major executable files it contains must now be digitally signed, which is an expensive and time consuming process. Second, the main package is delivered by a different set of download sites than the updates, with different bandwidth concerns, so the main package is kept as lean as possible. Finally, if the package did contain the detections, most would be replaced within a week, making almost all of the updates it contains simply wasted space.
This is why spybotsandra stated the package would need to be completely replaced each week, which my explanation should show is messy and costly.
Updating any of the regularly changing detections and other supporting files in an entirely independent manner makes sense, especially in an Internet delivery based system. In fact most Internet delivered antimalware software now installs this way, it simply does the initial updates silently in the background so you're not aware that it's even happened.
Bitman