Manual drag-and-drop does not scale well (a) with the total number of packages to be kept up-to-date, or (b) with the number of dependencies per package.
Dependencies are not bad - they are inevitible whenever resources are shared between applications (and will become much more common if we ever settle on a shared library system). A package manager encourages sharing by ensuring that dependencies do not complicate the installation process.
(That said, I am all in favour of keeping non-shared resources within the relevant application directory. I'm not trying to encourage packages that scatter files throughout the filesystem.)