More research - the inevitable incorrect/imcomplete requirements

The article: Estimating software projects(2001).
This article addressed the issue of the inaccuracy of project cost estimations due to that fact that it is done in the initial stages of the software development process and therefore likely to be based on incorrect or incomplete software requirements. More specifically it details the causes of poor and inaccurate estimation as being a result of imprecise and drifting requirements, new software projects are nearly always different form the last, software practitioners don't collect enough information about past projects, estimates are forced to match the resources available and there is a general lack of acceptance that developing software is an expensive endeavor. It went on to suggest cost estimation techniques and their various success rates.
This article lead me to the following questions: Does this mean that the completed, correct and non-changing requirement document which is need to deliver a successful project is never available at the commencement of a project? And that IT managers have come to expect this and concentrate their effects on the inevitable add ons and complications? Perhaps this is the reason why there is much focus on a project's contract and accountability. Or perhaps I have become too pessimistic :-D