Apparently not common enough
for evolution to have weeded out the let's-cleverly-gather-under-the-tree-during-the-violent-thunderstorm gene.
In all seriousness, you'd think that cows with a propensity to just lay down in the grass or mud when it rained would have a slightly higher probability of survival...unless these are the really dumb cows who accidently drown themselves at higher rates than the others who get fried by lightening...