I would also add that the Civil Service don't necessarily have all the expertise required to deliver these projects optimally. And they are also constrained by being in the public eye to ensure value for money and to be scrupulously clean in how money is spent - which is why large value contracts invariably go to competitive tender (and putting a tender together may cost companies 6 figure sums to put together - which has to be recouped if they win, and is lost if they lose. But assessing those tenders costs time, money and expertise, and contractors may be called in to help with the technical assessment.
Where the process can fall down is if the tender documents don't tie the requirement down absolutely or worse still, accurately capture the requirement - but although it may not seem like it, some Government departments are much better at doing that than they were 20 or so years ago.
BUT - this is taking the thread way off the original topic.