I can't find it ATM, but recently there was a good article that linked this phenomenon to the disappearance of unions. As unions disappear in the private sector they become increasingly dependent on the public sector to employ their laborers.
Because union members vote and because unions so visiblly represent the working-class, in many cities the best way to be pro labor is to support union labor.
In areas were unions are strong but increasingly relegated to public sector work, that translates to pressure to bloat public sector projects with union labor.
One way to think of it is as a form of work program.
On a related note, some conservative commentators have pointed out recently that if you combine U.S. healthcare spending with U.S. welfare spending, it roughly equates to what European countries spend on healthcare + welfare. IOW, we don't overspend on healthcare so much as we trade-off social welfare spending for healthcare spending.
Similarly, I wouldn't be surprised if you combine the overspending on public works projects with similar jobs programs, it would likewise roughly equate to what European countries spend on public works + jobs. IOW, public works projects are expensive because we refuse to adequately fund training, long-term unemployment benefits, etc.
I think this is all comes down to Americans' inability to embrace the necessity and role of social welfare programs. But because we can't admit of the need doesn't mean our society doesn't ultimately respond to the need; our response is just obscure and indirect in a way that preserves the fantasy of the free-market. For example, we can blame these excessive public works projects on "corruption" and "bloat" rather than admitting that they're pressure-relief valves for legitimate political, economic, and social demands. Something similar could be said regarding the Trump phenomenon.