Also known as the Plain of Storms, the Tomb of Taranis, the Graveyard of Thunder, and other colorful names
Lightning elementals are rare, the conditions required for a bolt of lightning to become self sustaining and self aware are very unusual and uncommon. As such, artificial elementals far outnumber wild ones.
When was the last time a character caught a cold or flu? Real people get them every now and again even with solid heating and dry interiors. Your characters are out in the cold and wet, and even inside is not always dry and warm. Imagine having to stop your dungeon crawl, sitting in a tight defensive position, while you all are feverish.