What I don't understand about preseason

Because they're losing 10-15 AD from the Runes Reforged system. There's no more innate AD, AP, magic pen, armor, health, CDR, etc. So AD champs need the bump to remain relatively functional in early game. With flat MR and MR/level glyphs being removed, AP champs actually come out ahead even with losing magic pen runes, so aside from HP/Armor buffs here and there they don't need anything else.