Shouldn’t it be more efficient to download only the changes and patch the existing files?

As people mentioned, that becomes problematic with a distro like arch. You could easily be jumping 5-6 versions with an update, with some more busy packages and updating less frequently. This means you need to go through the diffs in order, and you need to actually keep those diffs available.

This actually poses two issues, and the first one is that software usually isn’t built for this kind of binary stability - anything compiled/autogenerated might change a lot with a small source change, and even just compressing data files will mess it up. Because of that, a diff/delta might end up not saving much space, and going through multiple of them could end up bigger than just a direct download of the files.

And the second issue is, mirrors - mirrors need to store and provide a lot of data, and they’re not controlled by the distribution. Presumably to save on space, they quickly remove older package versions - and when I say older, I mean potentially less than a week old. In order for diffs/deltas to work, you’d need the mirrors to not only store the full package files they already do (for any new installs), but now also store deltas for N days back, and they’d only be useful to people who update more often than every N days.