So the highest flat armor pen is 18, not counting prowlers, which most people would take level 1 anyways. Usually 3rd item is around level 10-13 and by then most champs have at least 60 armor. The thing is, at 60 armor syrelda's has 18 armor pen, and LDR has 21, and they both only get better as the game goes on. So basically what's the point of going anything else as an assassin looking at a third item?
Because another item may be a higher effective value for the game. Eg. If they have a lot of shielders, then serpents fang may give more damage. Or if you are already over killing but just need to Snowball the map fast then umbral or youmuus would be better. Or if you need to avoid opening cc then edge of night may be better.
Just because an item has the highest raw damage for the situation doesn't mean it's the best item to buy.