Really? Are we sure we can't figure this out? Who believes that? Private contractors seem to be very expensive. No-bid contracts are obviously not competitive. And when the tax payer is picking up the bill, is there really any incentive for a contractor to keep costs low? Who advocates for the taxpayer? And how much corruption, kick-backs, lobbying, political favoring, porking, and other questionable activity goes on…
I think the question isn't "Why are infra costs so high?" which I think your list answers, but instead "Why are infra costs so high in the united states?"