AWS/GCE are not cheap. They're cost-effective for certain classes of use-case, but beyond a given scale, or with certain base-level requirements, you're way better off colocating a bunch of a machines.
My understanding is Netflix uses AWS for their infrastructure. Are they not on a similar scale as twitter with similar needs? Or is there a difference I am not seeing, perhaps a different use case. (This is a serious question, I am not doubting you.)
I believe that Netflix mainly uses AWS because their needs tend to surge dramatically at certain times, so that if they built their own worldwide server network, it would have to be vastly over-provisioned for when people get home at 7 PM and start streaming.
Plus they were a pretty early-adopter of AWS, and they have a ton of infrastructure running on it already.
AWS/GCE are not cheap. They're cost-effective for certain classes of use-case, but beyond a given scale, or with certain base-level requirements, you're way better off colocating a bunch of a machines.