Lethality is not a problem...

Why do people think flat armor pen itself is op, so every ad casters use it all the time?

I am sure the problem is that there is high attack damage stat combined with flat armor pen. Don't you guys remember old old Youmuu's Ghostblade when it's stat was 30 ad 15% crit 10% cd flat armor pen active : attack and movement speed bonus

I remember when I started Lol first time(in season 4), my Diamond assassin main friend says "Although, it has only 30 ad you need to build armor pen on talon, zed and khazix because they have high base damage and lower ad raio compare to jayce. thats why jayce builds tears instead"

but now.... Youmuu's Ghostblade 60 ad Lethality 10%cd

edge of night 60 ad Lethality 35 MR spell shield

why would jayce not use these items? why would jhin and varus use crit build that only have two 70 ad items without armor pen and one 0 ad with attack speed item?

edit: crit build im talking about is essence reaver + ie + as item