Why do people move between commuter train cars, even though signage explicitly prohibits it except in an emergency?

In Chicago, I’ve noticed that some people like to move between commuter train cars on the CTA “El.” All types of people tend to do this, young and old alike, for various reasons.

On the Metra, the gap between two cars is designed to allow passage while the train is in motion. However, the CTA “El” allows for emergency use only, as the gap between two cars is not enclosed.

When I see or hear people use the emergency exits while the train is in motion, it can be very startling and feel unsafe.

Why do people do this? Why isn’t the signage enforced? Also, is this a common occurrence in other cities with commuter trains and subway systems?