I think they also start earlier to build in more snow, ice weather days. Seems like even the treat of a snowstorm and school is cancelled or delayed.
I think in my whole 13 years of schooling living in Iowa at the time we maybe had 10 days off. Most years none. I think one year there were 3 snow days. We thought that was amazing.
Liability fears. Schools here are closed - the whole county - if there is a single bus route which cannot be safely driven. One evergreen-shaded back road is iced over, and all the kids stay home, and all the parents who counted on the schools' being open are up a creek. And I try not to chortle.