when can’t we create accounts to: sync bookmarks, passwords, tabs, history, payment methods and so on, across devices?
Pretty much exists under Sync.
Because that’s not as safe. They go end to end encryption and keep as little information of ours as possible. Creating usernames and passwords creates an extra database and higher chance of being hacked and information stolen.
Basically, you can sync devices with ease. True, it won’t serve as a “backup” but that’s a whole other discussion. It’s simple overall to create backups and store it in a safe place if that’s what you need. Sometimes security and privacy don’t equate to convenience.