Does this happen for official packages or just the AUR? I was pretty sure that when possible they build the packages from source themselves rather than bundling pre-existing binaries, and I'd assume that they wouldn't likely have the sources in the final package. IIRC Debian doesn't even like to bundle the headers in with libraries in preference of splitting them off into a separate file, so I'd be kind of shocked if they put the entire source of each library in the .deb files they distribute.
Arch package maintainer here; we generally encourage building from source for packages provided by the official repositories. As far as I know, we only ship pre compiled binaries if there is no source available, i.e. commercial programs such as reaper.