As Gravis walked through the Lightning Sect, he looked around. The buildings were made of Balzar, just like the Lightning Guild in the Middle-Continent. Gravis guessed that only a material like Balzar would do when building under a constantly raging thunderstorm.
One should know that natural lightning had around the same Energy density as someone at the ninth level of Energy Gathering. Lightning cultivators had an increased resistance to lightning, but that didn't make them immune to it. If some stray lightning bolt hit them, people below the ninth level of Energy Gathering had a very real chance of dying.
Not everyone inside the Lightning Sect could be a Spirit Forming Expert. There were just too many people. On top of that, they also had to teach the next generation, and they, obviously, didn't have enough strength to resist natural lightning.