Why would you need to run the Dropbox client to sync your passwords? Shouldn't the password manager client be able to talk to the service directly?

I don't have an iCloud client installed on my devices (well, other than whatever is built into Apple devices) but Enpass just uses their APIs to sync passwords. Occasionally I have to reauthenticate on one of my machines but it tells me as soon as it loses access. Previously I synced it over Google Drive and I never installed a Google Drive client either.

It's totally possible, but that sounds like it would cost a lot of development time. The KeePassXC project in particular decided not to implement cloud connectors due to a lack of resources.

I'm not sure why you think this is that difficult of a task. This is standard fare for how clients integrate with a service. There's how many third party Twitter and Reddit clients out there, plenty made by a single person that just communicates with a public API made for integration. You can spin up a Discord bot in a few minutes. Why is this any different for syncing a file over an API?

In my experience, sync conflicts are pretty common on SyncThing, too, which is often how I hear this paired ("use KeePassXC on Syncthing for the win" type deal)

I think it's an impedance mismatch trying to take a single-user database format and trying to use it from multiple devices