Google (astoundingly) has skipped any sort of syncing solution for its Android platform. Important items such as contacts, emails, accounts -- and even applications -- can be backed up to the cloud, but it's not quite the same thing as having a desktop alternative. While the Android Market, Palm App Catalog, BlackBerry Apps World, and Microsoft Marketplace for Mobile all have online presences, none of them can claim to have the same level of utility that iTunes does. Simply having an online presence is not the same as providing a separate piece of software that acts as a storefront and device manager.
Speaking personally, I'd say that I have downloaded 80% of my iOS apps not from the device itself, but from iTunes. The iTunes App Store is superior for discovery. It's far easier to search for and read about applications via iTunes on a computer than from a device. This is the greatest failing of all the other stores and why none of them will be able to match Apple's success in the near term.
Google has the best chance of gaining ground on Apple when pitted against, Microsoft, Palm, and RIM. It has the app selection in place -- OK, not 350,000, but still well more than 100,000 -- and the resources to deliver a solid piece of desktop syncing software for managing applications. Why hasn't it? Why haven't the others? Are the carriers standing in the way? Are the handset makers? It could be that there are too many hands in the pie. Apple controls the App Store experience, and doesn't let its carrier partners interfere with how it is run.
This will not be an easy challenge for Apple's competitors to overcome. Their ecosystems have more complications and parties with vested interests trying to make sure they get a cut. Apple's strategy here has clearly worked, but it remains to be seen whether or not Google, Palm/HP, RIM, and Microsoft can figure out how to mimic Apple's success.