datadir cannot be changed even when specified via CLI

[mariabackup]

datadir=/opt/galera/data

user=xx

password=xx

host=172.30.8.10

mariabackup  --datadir=/opt/galera/data   --backup  -u xx -pxx  --history --compress  --parallel=4 --compress-threads=4  --no-version-check   --target-dir=/data/backups/2020-12-05 

[00] 2020-12-08 09:31:10 Connecting to MySQL server host: 172.30.8.10, user: xx , password: set, port: not set, socket: not set

[00] 2020-12-08 09:31:10 Using server version 10.1.35-MariaDB-1~bionic

[00] 2020-12-08 09:31:10 Warning: MySQL variable 'datadir' points to nonexistent directory '/var/lib/mysql/'

[00] 2020-12-08 09:31:10 Warning: option 'datadir' has different values:

  '/opt/galera/data' in defaults file

  '/var/lib/mysql/' in SHOW VARIABLES

mariabackup based on MariaDB server 10.5.8-MariaDB Linux (x86_64)

[00] 2020-12-08 09:31:10 uses posix_fadvise().

mariabackup: Can't change dir to '/var/lib/mysql/' (errno: 2 "No such file or directory")

[00] 2020-12-08 09:31:10 my_setwd() failed , /var/lib/mysql/