Because it isn't a backup. They put the database into a quiescent state on disk, take a file system snapshot, let the dbms resume working, and send the snapshot

Because it isn't a backup. They put the database into a quiescent state on disk, take a file system snapshot, let the dbms resume working, and send the snapshot data via rsync. This requires the cooperation of the dbms software to get the on-disk data quiesced. Then your snapshot has to go fast enough that the dbms doesn't end up with too many spinning plates before you let it start writing normally.