You shouldn't have used authoritatively universal quantifier. There are plenty of sysadmins who use ES for this case, you apparently just happened to only be exposed to using it with websites.
Then, what ES+Kibana give me over grep? Search over specific field (my logs are parsed to a proper data structure), which includes type of event (obviously, different types for different daemons), a query language, and a frontend with histograms.
Mind you, troubleshooting around a specific event is but one of the things sysadmins do with logs. There are also other uses, all landing in the realm of post-hoc analysis.