Thing is, a large percentage of internet-connected users might have two or more devices. The simplicity offered by a cloud (be it hosted or selfhosted) password manager is a huge benefit.
And unless you’re already running a syncthing-like service for something else, setting it up just for a password manager when other services provide it out of the box, is not worth the hassle usually.
My setup is a barebones Alpine Linux with ssh and docker, and everything I run on it is a container (except backups).
Those I manage remotely (remote Docker context), so the only time I have to log in is to do an update for the few system packages and that’s it. And for that ssh is more than enough