The issue appears that your minimal OS installation/template is missing rsyslog:
# ps a x u | grep syslog
root 29810 0.0 0.0 112708 976 pts/0 S+ 01:50 0:00 grep --color=auto syslog
# rpm -qa | grep syslog
#
Install and enabled the service:
# yum install rsyslog
# systemctl enable rsyslog
# systemctl start rsyslog
Check
# ps a x u | grep syslog
root 30329 77.0 0.6 765392 108240 ? Ssl 01:51 0:06 /usr/sbin/rsyslogd -n
root 30765 0.0 0.0 112708 976 pts/0 S+ 01:51 0:00 grep --color=auto syslog
And now /var/log/maillog is being populated:
# cd /var/log/
# stat maillog
File: ‘maillog’
Size: 2541357 Blocks: 4968 IO Block: 4096 regular file
Device: 98d0b6b1h/2563815089d Inode: 9660 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 47/mailnull) Gid: ( 12/ mail)
Access: 2019-03-21 01:51:29.483114937 -0600
Modify: 2019-03-21 01:51:29.478115093 -0600
Change: 2019-03-21 01:51:29.478115093 -0600
Birth: -
#
Please note that cPanel does not depend on rsyslog for any sort of logging as this is the operating system's logging daemon, which is why it is not installed during the cPanel installation, even if it is missing after the OS installation. You will need to ensure this package is selected during the OS installation or installed in the VM template.
I also want to note, that a few years back there were issues with rsyslog running on OpenVZ containers causing high CPU usage, which is probably why the service was not installed into the template:
openvz rsyslog - Google Search