why I can't see the syslogs in opevents but yes in nmis?
NMIS displays syslog entries just as a visual courtesy to the user, but it does not consume or handle them i any way.
opEvents on the other hand is meant to analyse and react to problems and issues and therefore consumes only specifically configured log files and input sources whoose format is known or where the administrator has created custom parsing rules.
You can configure opEvents to use and act on arbitrary log files (incl. syslogs), but you will need to configure opEvents to tell it what log entries constitute 'interesting' events in your environment and how to react to them. This is documented on the opEvents input sources and the Event Actions and Escalation pages.