It partly has to do with the bidding process - the drive to get the lowest bid. Competent companies know the cost won't work and are unwilling to put up the surety bonds to take on a project. It leaves with the boldest incompetent company to make the lowest bid who gets their foot in the door and then jack up the cost overrun later on. Often the project failed due to cost overrun or sheer incompetence. At the end the cost of doing business go up for all parties involved.

I've seen transit projects with hundreds of millions budget fell apart with nothing to show at the end, and have seen a transit project that doubled the cost and tripled the schedule to get to completion, and that was a good project.