Because they don't want to stop. On long trips, my wife and I like to drive straight through the night, and have been known to have our boys pee in a jug rather than trying to last until the next exit.
We don't leave the jugs by the side of the road, though.
I do think a significant part of this phenomenon is caused by migrants (can you say illegal?) who are accustomed to this method of elimination and who will leave the waste at the roadside because they do not respect our society, or look upon it as a mere extension of their own. I used to own rental property in San Diego. The offspring of illegals, lately come from living in a cardboard box, would amuse themselves by beating down the hedges out front with metal rods. Illegals would enter garages and defecate on the floor or on the sidewalk. I hasten do add that there are American low-life slobs as well.
Dad, what the hell are you doing on this forum!
Stop, Zip, and Go. Peeing intransit just doesn't save that much time. Now days plenty of places are open 24hrs. :)