That's because the whole Android API and ecosystem was designed to make it possible.
You cannot get away with that and POSIX applications. They have zero or bad session support. There's no global always available database to help you (Android has 4), nor event scheduling bus. (Dbus is a joke, it does not allow sending event to non-existing endpoint to be delivered later.)
Every application using the standard mechanisms will get relaunched and activated as needed if it got shut down. It will receive a Bundle with state it managed to save and with original launch Intent. It can access an sqlite database to be made available via a restartable ContentProvider. SharedPreferences are also stored. Etc.
In Linux world, there are no such de facto standards and what is most common is utterly broken.
Same in Windows (only registry is persistent, it's not meant for data storage) and in OS X.
