No matter how much armor pen can get silly on AD casters etc, it still will have issues.
An all AD team will fall off later game with no AP present, the opposing team will just ignore MR for the most part and build health + Armor, it's enough to get them buy.
AP casters bring more utility than ADs usually will. Morg, Anivia, Ahri, Orianna, Zyra, Fiddlesticks, Twister Fate, Lux, etc.
They are going to bring more to a team than Talon, Pantheon, Lee Sin, Rengar, Wukong etc. will in over picking one AP caster over them. Some AP might have less utility but still have a place the ADs can't fill.