Why do big IT projects fail?

It probably doesn't matter which country you live in you'll be familiar with news stories of large government IT projects that have either failed completely to deliver or have gone seriously over budget and resulted in numerous bugs. Listen to this recent radio interview by University of Auckland Computer Science Lecturer, Paul Ralph; he says "when it comes to the public procurement of big software design projects - don't! He says the total cost of larger IT projects always cost more than everyone thinks, they're almost impossible to accurately forecast in advance, and innovation is constantly undermined by the tender process."