Typically, if the benefits of a project exceed the costs people will build the project without government involvement, in a mysterious process called the free market, but more accurately referred to as freedom.
That leaves the government with nothing to do, unless they can dream up a lot of projects whose costs outweigh the benefits. But people wouldn’t vote for those projects unless the politicians lie and claim the benefits are much greater than they really are and also underplay (and, where possible, underpay) the costs.
Not to mention that some politician grow wealthy somehow in the process.