iOS makes it painful to use third-party sync protocols and servers, like syncthing can't run in the background, a git sync service can't run in the background, only iCloud gets to run in the background.... and whatever sync protocol the app itself has blessed so it can run immediately on opening the app.
As such, on iOS the native sync is the only one that works cleanly and seamlessly, and so you're incentivized to pay for it.
There was a little while, when dropbox was big, where it seemed like the future of computing would be "your data is in the cloud, and every app you use can share that data, and those two things are independent integrated through some common filesystem layer".
And then it ended up that no, your data's in a cloud-per-service, where your emails live in googles cloud, your documents in microsoft 365's cloud, your images in "adobe creative cloud"'s cloud, your photos in Apple's cloud, your passwords in 1Password's cloud, and your knowledgebase in Obsidian's cloud.
The dream of the filesystem API being able to expand to clouds, of being able to choose dropbox or google or apple as the owner of your data, and other applications seamlessly integrating with any of them, it died with apple making it impossible to offer any sort of generic filesystem API or even background sync.
And so, that's why you'd use obsidian sync over git, because you're cursed with using a phone.
Unless you're saying "why not pay for obsidian sync, but then sync it into a git repo in CI and commit there to see the diffs", not "why not use git as the underlying sync protocol", in which case ignore everything I wrote, you totally could do that.
> it died with apple making it impossible to offer any sort of generic filesystem API or even background sync
Apple's cloud storage remained WebDAV a very very long time.
Apple's iOS has a pluggable Files system. Use Working Copy to give other apps access to folders sync'd with git: https://workingcopy.app
Or a dedicated app like GitSync: https://apps.apple.com/us/app/gitsync/id6744980427
Gotcha, thanks. I just use git but don’t sync to my iPhone, this helps give context on the value prop there.