> Wearing a helmet in a steel cage seems kind of redundant.

All kinds of race car drivers were helmets - and their steel cages are much much more sturdy than the ones that commuters have. Statistics als indicate that many car drivers suffer head injuries. So by all available evidence, wearing a helmet in a car is not redundant. Still, people don’t do it - unless they engage in particularly dangerous activities.

The parent poster that you refer to explicitly gave his speed as about 20km/h for commuting, which is neither particularly fast nor particularly dangerous on good infrastructure. So why wear a helmet?