You also need a CDN. NetFlix deploys boxes all over the place to cache the content.

I agree there's nothing in there that's not understood from an engineering/technology perspective. It's not fusion or curing cancer. But scale and complexity compound simple engineering problems. There monitoring, there's billing, there's machine learning, there's software lifecycle, multitude of different devices types and networks, all of that at extreme scales that need to work pretty reliably.