People who have looked into the question have collected a range of fairly convincing explanations — though they come up short in a fundamental way. Let’s quickly go through the major factors researchers have identified, in no particular order:

1. Expensive labor. From the top brass at New York’s Metropolitan Transportation Authority: “The MTA is required to overstaff projects so that the same [tunnel boring machine] work, for instance, that can be done in Spain with nine workers must be done in [New York City] with 25 workers.”

2. Out-of-control private contractors. From Stephen Smith at Bloomberg: “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.”

3. A crap procurement process. The classic American way to pay for a big project is to round up about half of the funding (or even less), start construction, and then use a sunk-cost-fallacy to get the rest. This, obviously, is not conducive to efficient or speedy projects. (Looking at you, California high-speed rail.)