Let's turn this around again. Why couldn't the expensive & extensive enterprise level intranet platform be less rigid?
If the platform would just provide the features and connectivity (ways to integrate to other systems etc), and be UI agnostic, wouldn't intranet projects be more about solving each organizations own specific problems? Instead of trying to make a rigid system bend, and the client with it.
Of corse, the realist in me knows that there is a certain mentality that goes with the "enterprise" world, and that Sharepoint often gets chosen.
Oh and, being UI agnostic doesn mean you couldn't have some defaults for the cases where no "creative digital agencies" were involved :)