I'm sorry, but just do the math. How much AP do you get in a Soraka Mid build, typically? 'cause a Soraka Support typically get 30% increased healing + 80 or so AP, not even full build, just with her core items. For her R Lvl11, for instance, that means going from 250 to 380. You'd need 235 AP to get the same result. The Idol items are much more than a band aid. And we didn't even talk about their unique passives yet, that synergize much more with Soraka's kit than mid items.
Also, "not everything's about damage", that's why each role/position is specialized. But if you don't have damage mid, who's going to pick up the slack?