The 3-2-1 rule exists because every single backup method fails in some scenario. Three copies of the data, on two different kinds of media, with one stored elsewhere, covers deletion, hardware failure, theft and fire.
Sync services are frequently mistaken for backups. They mirror the current state, so a deleted or encrypted file propagates to every device within seconds. Version history helps, but only within its retention window.
External drives fail too, usually without warning and often while sitting unused in a drawer. A drive that has not been read in two years is an assumption, not a backup.
The rule only works if it is tested. Restoring a handful of files every few months is the difference between a backup and a hope.