And also because it's not the only variable at play. Considering the variable "protection against the elements", one of the groups have cars as protection, while other have mere shoes or bikes. So different variables can be combined to try to balance things out.
Nice point. Made me think about people who have a choice to drive or walk. If the sidewalks aren't cleared they may choose to drive. This could lead to congestion for drivers. It could even be that people who can only drive benefit more when the sidewalks are cleared first.