Also, its not a hard and fast rule, just their best guess recommendation - they acknowledge there are a lot of factors involved.
But mainly this...
"...experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy."

The idea being that helmets in 5 yrs will likely be more advanced, just as the latest generation MIPS helmets today are (arguably) better than what we had 5 yrs ago. Of course, this assumes the industry is actually pushing the technology and not just working on new vent designs and paint jobs.