Why do applications have to be packaged for each distribution?

Question

Hi,
Sorry this is probably a stupid question, but i don't understand why it's necessary for each application to be packaged (and patched) by each distribution.
Example: (as i understand it) Mozilla Firefox. Arch has it's own version derived from Mozilla's, and so do the other distributions with their own variations.
Is anyone able to explain how this works and why it's done this way?
Bonus points: If anyone has a link to an article explaining in detail, would be very interesting to read

Answer 1

When packaging software there are a bunch of questions you're facing. Many have been mentioned before.
This starts with the compile-time configuration regarding which parts of the software should be compiled/used, eg some libraries can use different backends and you can only choose one. Sometimes it's even a matter of licensing. Some distros like to enable experimental features if they don't break the rest of the software, some let you choose (like gentoo's useflags mentioned above).
Filesystem layouts differ: Some prefer /usr/lib/pkgconfig, some /usr/share/pkgconfig; some like /usr/libexec, some don't, some (eg FreeBSD, which isn't linux but with regard to package management faces the same questions) use /usr/local for packages. Some distinguish between binary-only packages by putting them into /opt, some don't. Some actually have /bin, /usr/bin and /usr/local/bin as separate directories
Or you might go a completely different way, too, nobody's forcing you to stick to "standards". I'd also like to point to haiku, also not a linux distro, but there too you have most of the same software packages available, which have to follow different directions there, too.
Default configurations: Some get patched (themed for the distro or for other reasons), some don't.
Init systems: Gentoo (at least last time I checked) defaults to openrc. Most people do not provide openrc scripts in their source repositories, so the packager is responsible for creating these.
Gentoo actually brings up another thing: it allows for some packages to have multiple versions installed, and you can use them explicitly and/or switch your default with `eselect`. (It probably does more than that, I haven't used gentoo in years.) For this of course you need to package your software in a certain way. Eg put your binaries and libraries into a separate folder and symlink them to their desired locations, rather than installing them there directly, since they'd be overwritten by the other versions of the package...