I remember when a $100M public project was considered big. Now the minimum for anything consequential cost $1B or more.
1) When you put off work, it costs more to do it later
2) US costs are inflated relative to peer countries
3) Some of this is related to how we force additional, extraneous costs onto projects (ie, replacing road, you also need to replace a bunch of other tangentially related infrastructure as part of the project)
4) some of this is on excessively long permitting times
5) reliance on consultants and contractors because of the loss of state capacity (tax cuts and manpower cuts means no budget to keep in house experts
6) stop/start construction - we should move teams from one completed project to the next to prevent brain drain and having new teams constantly relearn lessons from old projects, instead of plan, mobilize, construct, demobilize, plan, mobilize...
7) some costs because of how some people insist that you cannot possibly impose a disruption on daily life for a big project (for example, building deep subway lines instead of using cut-and-cover techniques, or not allowing 24-hour construction)
8) and in some cases, union rules that mandate excessive amounts of people (ie, staffing for tunnel boring machines in the US vs Europe)