Why dont adcs get Spirit Visage as a defensive item when they play with Soraka?

Probably because it's a full item slot that, outside the small healing increase, is pretty much wasted on an ADC.

as a def item its not horrible but when you are trying to survive cc or burst its not the greatest either... gives hp , cdr, mr, and increase regen and healing. when it doesnt shield or block cc ect. now those stats alone can help you stay alive longer and make your lifesteal from stronger. which could be enough to stay alive to out duel someone.

I'm an idiot and was never aware that it increased healing for outside sources.

Learn something new everyday.

Well, it's pretty costly, so, unless you are really ahead, or it's one of the items you build after your core items, it's not something you should pick up or need.

Because it gives no AD and ADCs don't give a shit about building survivability outside of items like Maw or Mercurial or lifesteal? (And GA in late)

I play graves and build this

if the enemy team has 3-4 aps. i never die. unless i get focused by the whole team.

Spirit visage by itself won't make you live any longer since you're not building any other defensive items. Soraka should be able to heal you in 1-2 heals lategame.