Network traffic monitoring is routinely done at enterprises. It's usually part-automated using the typical approaches (rules and AI), and part-manual (via a dedicated SOC team).
There are actual compromises caught this way too, it's not (entirely) just for show. A high-profile example would be Kaspersky catching a sophisticated data exfiltration campaign at their own headquarters: https://www.youtube.com/watch?v=1f6YyH62jFE
So it is definitely possible, just maybe not how you imagine it being done.