We may not be able to resolve the age-old question of why, but we can address how. Drivers may not always see pet chickens crossing roads, so the company Omlet developed these shiny vests to enhance the visibility of chickens.
The company developed the vests after inquiries from prospective customers. Johannes Paul, one of the company directors, says that chickens tend to travel to roosting spots in the evening, which is often a time for peak traffic but diminished visibility. The vests attach with Velcro straps and include an insulated lining to help keep chickens warm during cold nights.
P.S. Bonus item from the webcomic Man Eggs: