Most of the studies I have read chalk it up to two factors. First is changing project scope. The regulatory and political environment in the US means that projects often go through several planning changes, introduced in the middle of development, leading to wasted resources not just to overcome red tape, but executing on the plans that become obsolete.

The second that I have seen is codified labor specialization. This leads to many more workers on a site, where each has a niche role, but low workload.