The problem with taking Google maps as an example is that it's proprietary to Google. There is no public library (that I'm aware of anyway) which a developer can use to interface with the google maps/earth repository. So unless Google would release a public library which allows access to their maps or develops their own app for Maemo or Qt, we won't see a native google maps app I'm afraid.

Qt is fundamentally a user interface development library which allows developers to easily code the user interface layer of the application. (It also includes libraries to interface with databases, files, network etc, but those are secondary to the user interface components) Without Qt (or another cross platform library), to develop an app on multiple platforms you'd typically have to develop the user interface separately for each platform. Each platform will have a different set of calls to put up a button for example. With Qt, the calls to create that button are the same for each platform so you only have to develop the interface layer once and it is portable across the other platforms supported by Qt.