two word answer: regulator capture.
short answer: American system of preferring private enterprise and gutting government and public service means that over the years, a handful of big private contractors have figured out how to gave the system for private profit at the public teat.
So says, in essence, a couple of articles-- in Bloomberg, and in Mother Jones:
U.S. Taxpayers Are Gouged on Mass Transit Costs - Bloomberg
A huge part of the problem is that agencies can’t keep their private contractors in check. Starved of funds and expertise for in-house planning, officials contract out the project management and early design concepts to private companies that have little incentive to keep costs down and quality up. And even when they know better, agencies are often forced by legislation, courts and politicians to make decisions that they know aren’t in the public interest.
Civil Engineering in America Is the Most Expensive in the World | Mother Jones
…Littlefield also argues that judges in New York routinely side with contractors in disputes with the Metropolitan Transportation Authority. "In the private sector, if you rob your customer, you will suffer a hit to your reputation and possible losses in the courts," he said in an interview. "Not so if you rob an agency like the MTA. Then it's all rights and no responsibilities."
The MTA must continue to award contracts to the lowest- price bidder, and without the ability to hold bad contractors accountable, Littlefield said, the agency turns to "writing longer and longer and longer contracts, expressly prohibiting every way it has been ripped off in the past." The byzantine contracts that come out of this process drive entrants away, limiting competition and pushing up costs.
i'm sure someone will shout, "Unions!" but then Europe too has unions. American unions are that much worse in their work regulations?