Check out the price tags on most infrastructure projects these days. As badly needed as they are, the US government currently doesn’t have re ability to execute them for less than the mid-horizon returns, if that.
Secondarily, many prospective borrowers of these projects are already in debt, and have cash flows which are not growing fast enough to borrow more.
why should a gov't project need to have any real returns? Social returns is enough. If the city is nicer to live in, if the businesses thrive because of increaed foot traffic, lower car accident rates, cleaner air etc.