Why can't I download apps from the Android Market via my PC?
Many developers have their own sites and there are other app markets that allow direct download.  But besides that, we can't speak to their motives, you'd need to ask them.  I would assume, however, that they're controlling distribution in order to control distribution :P.  For paid apps this is obvious (piracy prevention), but even for non-paid apps there are developers and others who want to monitor, limit, etc. the download of apps.