It's shocking how many Git repos are used to build this project. It seems unnecessary. Personally I find the ability to make atomic commits across different parts of a project very handy. Don't understand the drive to break it into so many parts.
But it's also just a huge ecosystem with parts that are used and developed independently. E.g., "translators" for save/import/export are used in both Zotero and in the Node-based translation-server (used by Wikipedia and others), and we can give commit access to those separately from the core code.
(Disclosure: Zotero developer)