Android apps always part of the OS source?

Android apps always part of the OS source?

I am trying to customize the Email app that's at packages/apps/Email in
the android source. However, I noticed that it has a dependency on the
OS/SDK that it's part of and is not really separate.
For ex., if I pick up the Email app from the latest master, it will run
only on 4.3 android devices, and not on 4.0.x. The app uses certain
features of device admin that are there only in 4.3 and is not written to
work on the previous versions.
And if I take the app from 4.0.x android source then I am missing the
latest and the greatest of the Email app that's in master.
Is there a reason why the apps are not maintained independently of the OS,
and built to be backward compatible?
Thanks.