There are multiple things that are done here. Suppose you had great, immutable backups. They still have many things that can ruin your business

1. Restoring networks, servers, third party services with knowledge that anything you restore could be compromised as well. Keys

2. The attackers will then threaten to dump all of your private information.

It is more than just restoring data, it is restoring and resetting your entire infrastructure. And most places have backups, but they don't practice entire restores