this sounds more like a problem on the side of the publishers than itch
This.
No one forces a developer to split the project into 17 and a half files. If they are small, why not offer a zip, plus the individual files.
(From a technical aspect, a download all button does not work. It would only be possible to be a workaround, where the server just packs everything into one archive, but the server does not know about redundancy. This is the work the publishers should do. If they split it into many tiny files, it is their fault.)