The answers given are the obvious one; that pre made content can be buffered heavily. But the internet can, at its core manage packet delivery within the delay

The answers given are the obvious one; that pre made content can be buffered heavily. But the internet can, at its core manage packet delivery within the delays required for real time communication. More precisely the issue is probably one of supply and demand; that our internet is tuned for content to radiate out from large scale producers of content, flowing down to its consumers. There's far less capacity and focus "laterally" in the network -- between consumers and other consumers -- as is required by VoIP sessions.