How do you propose to sync a file without a server? What do you do about conflicts? Sure you can do this, but IMO the experience sucks in comparison.
Even if one needed a server for syncing files, should each application that needs synching require its own server?
But I think in this case it's pretty much required for good UX with the way file syncing works on mobile and in the browser, especially dealing with conflicts. I've done it both ways and there is a lot less friction with vaultwarden than my old 'synced KeePass file' approach.
It was just a suggestion, there are other ways to skin the cat.