Because in my experience it's the only save icon that's consistently used for the same purpose. Many apps use icons involving arrows for most common actions, and then I have to guess if that one icon is for downloading something, for saving the file or something else entirely.

Smartphones really need a tooltip replacement. Windows Phone 8 solved it in a consistent, intuitive way and it would be great if the other big players came up with something similar. Google and Apple have the power to push such improvements through the whole app ecosystem, but there seems to be no incentive.