Labor costs rarely make it into the double digits when it comes to percent of total costs of large infrastructure projects. And even if they were, that wouldn't explain the discrepancy. The infrastructure construction costs in very first-world Western Europe are closer to the costs found in India than they are to the costs found in the US.

If it's not labor, where's all that extra money going to for US projects then? Materials? Legal fees?

To say it's not labor is not entirely true. I'm just claiming that labor isn't a huge part...at best it contributes in the single digit percentages.

The real answer is much more complicated and there is no easy one-size-fits-all answer. Environmental regulation, litigious environment in the construction industry, overuse of contractors and consultants instead of in house expertise, buy america provisions, labor scheduling rules, poor planning, supplier market that has been drastically cut off due to bidding complexity and procurement regulation, bad incentives, poor oversight, corruption, incompetence, etc ad infinitum. If you had to sum it up, you could say that the US is uniquely incompetent in all aspects of government-sponsored infrastructure projects.

A lot of it has to do with byzantine regulations.

Mostly legal haggling about land ownership and various rights like that.

Try: right to the top.