Explanation: Not cities in general, but the city of Rome, specifically.
https://en.wikipedia.org/wiki/Secessio_plebis
These secessions happened when Rome was still a single city rather than a massive, sprawling polity, so labor mobility was at a minimum. Lots of borders, both political and cultural. The seceding workers could not be replaced on short-notice, even at increased cost, or by temporary hires. No sending messages to a hostile neighboring city-state asking "Pretty please let us hire some of your filthy poors, even at double normal wages 🥺"; you have to work things out with the workers you have before the crisis devours the entirety of your wealth!