Why are there magic pen boots but not armor pen boots?

I think it has more to do with boots being designed for the jobs of particular roles.

Marksmen generally rush AD and build towards sustained, so having boots for a quick bit of AS is nice.

AP Mages generally build AP and go for burst, so Magic Pen boots are nice.

Bruisers/Tanks who are generally melee and build the damage they need, can benefit pretty nicely from some early defenses armor/mr really helps.

Mobility Boots and Swiftness are nice for junglers who need to get around quickly or Champs that are really hurt by slows.

I could definitely be wrong though. Armor pen boots sound like a good idea, but giving away a stat like that cheaply (with t2 boots) might make AD champs a bit overpowered early/mid game.

there are more viable Armor pen items for straight damage than there is for magic pen