No one would ever bid this. This is classic time and materials work, too much volatility for anyone to bid it as lump sum. At least, not without charging a MASSIVE mark up to CYA.
By massive mark up I mean multiples more than what is initially expected to be needed, and even then the provision for change orders would be demanded if the scope goes too far out of the original bounds.
In other words, you'd be creating a lose/lose scenario for the gov't. Either the job stays more or less within scope and the private contractor makes a killing thanks to a fat number or the job goes way out of scope and the gov't. gets change ordered to death like it was a T&M job but with the initial overbid effectively turning into a "bonus".
Another problem: Almost no companies exist that can handle this kind of work, and those companies don't spring up overnight. There is a reason why Halliburton milks gov't. contracts so profoundly, because in a lot of cases they're the only company that truly has the supply lines and corporate resources to do a job. They know this, and they make the most of it. Hell, they're so damn big you literally can't build a TON of shit in their sector to code without buying Halliburton products.
Same with Jacobs responding to the vast majority of post-Katrina clean up. They knew they had a blank check and took full advantage.