Few reasons, with the most important being convenience. Syncthing is going to see just a binary blob as the password storage is encrypted. This means it is impossible for syncthing to do proper synchronization of items inside the vault. Generally this is not a problem, but it is if you happen to edit the vault on multiple devices and somehow syncthing didn't sync yet the changes (this is quite common for me on android, where syncthing would drain the battery quite quickly if it's always actively working). For bitwarden on the other hand the sync happens within the context of the application, so you can have easy n-way merge of changes because its change is part of a change set with time etc.

Besides that, the moment you use syncthing from a threat model point of view, you are essentially in the same situation: you have a server (in case of syncthing - servers) that sees your encrypted password data. That's exactly what bitwarden clients do, as the server only has access to encrypted data, the clients do the heavy lifting. If the bitwarden server is too much of a risk, then you should worry also of the (random, public, owned by anybody) servers for syncthing that see your traffic.

Keeshare from my understanding does use hosting, it uses cloud storage as a cloud backend for stateful data (Gdrive, Dropbox etc.), so it's not very different. The only difference would be if you use your private storage (say, Synology Drive), but then you could use the same device to run the bit/vaultwarden server, so that's the same once again.

The thing is, from a higher level point of view the security model can only be one of a handful of cases:

• the password data only remains local
• the password data is sync'd with device-to-device (e.g. ssh) connections
• the password data is sync'd using an external connection that acts as a bridge or as a stateful storage, where all the clients connect to.

The more you go down in the list, the more you get convenience but you introduce a bit of risk. Tl;Dr keepass with keyshare/syncthing has the same risks (or more) than a Bitwarden setup with bitwarden server.

In addition to all the above, bitwarden UX is I would say more developed, it has a better browser plugin, nice additional tools and other convenience features that are nice bonuses. It also allows me to have all my family using a password manager (including my tech illiterate mom), without them having to figure out anything, with the ability to share items, perform emergency accesses etc.

Edit: I can't imagine this comment to be deemed off topic, so if someone downvoted simply to express disagreement, please feel free to correct or dispute what I wrote, as it would certainly make for an interesting conversation! Cheers