Look at the permissions:
ls -ld /var/lib/mysql. You most probably don't have read access to the raw tables as a regular user. This is by design: these directories include sensitive user data.
You should either run the backup as root, and/or even better: use a database dumper scheme such as
automysqlbackup or make your own with e.g.
mysqldump and a small script.
It is not recommended to back up the raw data directories themselves, since it can be problems if you back up while a transaction is happening, and other risks of corrupt backups and subsequent restore issues. The way to do it is to ask the MySQL server to dump the data for you in a chosen, easily restorable format.