Re: Why?
Pretty much any non-trivial project will be over budget, late and buggy if the project manager and the technical people who design it, manage development and run unit, integration and acceptance testing are not part of the bid team and their inputs are ignored rather then used to determine pricing, staffing and timescales.
If the project is an in-house one, then a senior user should be part of the design and management team..
Unfortunately, civil servants and other people running governments and the like never staff projects this way and have no clue why it would be a good idea to do so.