[pacman-dev] Why can install files not be part of the sources?

[Dominik Fischer]

When a PKGBUILD specifies an install file that has to be downloaded as part of the source array, makepkg complains about the missing file instead of simply downloading it.

The wiki page [1] states that ".install files should not be included" in the source array, but I could not find and explanation why this policy is enforced. To me, it would make sense to point to an external installation script, for example to install desktop files, or maybe an upstream maintainer provides a suitable file directly. Is this an implementation issue or an explicit decision? Why are install files required to be locally present?

[1]: https://wiki.archlinux.org/index.php/PKGBUILD#source