Honestly their are just too many competing philosophies when it comes to supports.
No Tank support should be rewarded with Damage for building tanky. (not directly anyways) Putting that kinda damage on a tank, would push them more towards lane or Jungle, because it would allow them to farm. And because supports are gold starved they need all the defensive ratio's they can get.
Mage supports is another issue entirely, since you also have a crowd that "still wants them mid lane" In which, Higher base and lower ratios would be counter to that. That said, You can compensate lower ratios with more magic pen, and magic pen is pretty budget, and more appropriate for a support build. (meaning, it would likely make things worse)
Buy and large, higher AP, lower base exist on mage esk supports to balance between both roles. Because of the gold deficit. Riot pretty much writes off "Full AP Support" on the grounds that it "Shouldn't happen."
Another philosophy with support scaling is mini games. You get champions like Bard and thresh, who are rewarded from being the support and doing support things. And Riot doubled down on this approach with the quests on support items.
