The point was that when private enterprise cannot find an efficient way to create capital through productive endeavors, the government can. This example is not a crux of Keynes general theory. This example is to point out a key problem with free market economies; that due to uncertainty and risk aversion, venture capitalists are hesistant to invest. In this example, entrepreneurs and private enterprise would be foolish to not employ the tools the free market affords them to draw this capital from the ground. I find it funny people find the idea of extracting something valuable from teh ground that has little true utility that silly of an idea; people do it all the time with gold and diamonds. The demand for gold and diamonds provides the incentive, just as the demand for bank notes would provide the incentive in this case.