Because they work at a standard level of abstraction (the file system) and can be replaced if necessary. I can replace Dropbox with Box or Syncthing or OwnCloud or rsync or sftp coupled with crontab or an FSEventsD monitor or whatever I want, at any time I want.

1Password's proprietary sync isn't open source and isn't open standards. It is designed to lock me into reliance on a company that has inarguably demonstrated that it hates its users.

My password manager works just like this! It just monitors shared folders and keeps everything in a compressed/32k encoded XML file that you can open with a text editor.

It's 1 file per vault, but multiple people can use the same vault and all are update simultaneously.

I’m curious about this too. 1Password used to sync to a file that could be backed up to iCloud/Dropbox/drive and I wished they’d kept it that way, but it at least partially suggests they may have stuck with full client-side encryption (I have not verified)

Yeah they still claim to have full client side encryption for the cloud service. Maybe it's the subscription pricing model that's the issue for people?