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.
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.