What is the reason behind runes (especially offensive ones) having higher bonus AD than AP scaling?

All AD scaling champs get the inherent benefit that their auto's are all enhanced by the bonus AD as well as their abilities while almost all AP champs have basically all of their damage scaling loaded into abilities, with a select few having auto enhancements scaling off of their AP.

I still remember part of the reasoning for the change in 6.9 to turrets was to allow the AP champs auto's to scale off of their AP was, "On the other hand, we want more champions than just marksmen to interact with the turret killing game. We’re rebudgeting turret defenses so that other champions will feel better about their ability to play demolitionist." Maybe it is time to look into applying this effect to all auto's for champs.