With these systems, the runtime guarantees of data integrity are very high and the failure rate is very low. And best of all, failure is constantly happening as a normal activity in the system.
So once you have data integrity guarantees that are better in you runtime system than your backup process, why backup?
There are still reasons, but they become more specific to the data being stored and less important as a general datastore feature.
Because of mistakes and malicious actors...
Redundancy and backups are not the same thing! There's some overlap, but treating them as interchangeable will occasionally result in terrible outcomes, like when a config change that results in all 5/5 datacenters fragmenting and failing to create a quorum, then finding out your services have circular dependencies when you are trying to bootstrap foundational services. Local backups would solve this, each DC would load last known good config, but rebuilding consensus necessary for redundancy requires coordination from now-unreachable hosts.