Armor Pen in LCS

I have noticed a lot of AD players mid lane or bot lane not build armor pen. Even when the enemy team stacks armor and health. I am not sure why, I guess I would need an explanation. In the UOL vs FNC game on 2/12/2016 there were no Armor pen items on the ADCs. They looked like they were barely touching the front line. Especially on the side of FNC. Wouldn't somehting like on or be amazing for a game like that one?