I think it's just one of those things that sorta sounds right so people believe it. When you drive up the interstate you see cows up walking around so if someone says cows sleep standing up and you don't know any better then the rest of the premise also sounds reasonable since when it is dark you can't see from the interstate how the cows are sleeping.
A non-farm friend once asked why when he sees cows in a field by the interstate they are all headed in the same direction.
Cuz