1. The Department of Skunk Placement
Our tax dollars have to go somewhere, don't they? I bet with the surplus money they fund Government programs such as the the Bureau of Sock Misplacement, where men dressed in black sneak into our homes at night and steal one of each sock, and then the Department of Skunk Placement where every night a group of guys in environmental hazard suits take there genetically engineered dead skunks and lay them out in the middle of the road where people will smell them. This would explain why they are always in the median. I mean if people actually accidentally hit them they would be in the middle of the lane wouldn't they?
2. Stealth Skunks
Skunks have the innate ability to turn invisible which is why we never see them. Unfortunately, due to the way the ability bends light it also renders them blind, leading them to the tendency of wandering out onto roads where cars, which can't see them because they are invisible, run them down. This would also explain why skunks get hit in the first place. Who in their right mind would actually run a skunk over knowing that they are going to have to drive back that way the next day and have to smell it?
3. Skunks are Members of the Lemming Family
Many people don't know, but there are actually two different types of Lemmings. There are Daredevil lemmings who all get together and see who can jump the furthest and who can swim the furthest. Then there are the suicidal lemmings who figure there is nothing left to do in their mediocre lives of sitting in holes eating roots and decide, what the heck. Skunks are like that too... There are the Daredevil Skunks who say, "Hey Pepe, bet I could cross the road before that car gets here." and then there are the suicidal skunks who say, "Nobody likes me just because I'm a skunk! The world stinks!" They then proceed to throw themselves out in front of moving vehicles.