What are the perceived benefits of not bothering to be seen, and what are the disbenefits of wearing reflective clothing etc?
Aside from an 'arms race' between different road users that pedestrians and cyclists cannot ever hope to win, you normalise the fact that everyone has to dress up to be near a road. This means that walking, horse riding and cycling becomes marginalised and seen as dangerous activities and woe betide the person who was not dressed in high vis who dared use the road. It would be far better for drivers to drive safely so that vulnerable road users are not forced into modifying their actions for the selfish others. You can see how it is heading, there are already plenty of projects for vulnerable road users to wear a beacon to alert drivers of their presence, I can just see the first court case where the driver is acquitted because the vulnerable road user had a flat battery on their beacon and was therefore not detected by the system.