That's because we only allow ourselves to fund public spending through debt. In addition, if I may quote myself quoting Keynes:

European Tribune: Keen on Keynes

When involuntary unemployment exists, the marginal disutility of labour is necessarily less than the marginal product. Indeed it may be much less. For a man who has been long unemployed some measure of labour, instead of involving disutility, may have a positive utility. If this is accepted, the above reasoning shows how 'wasteful' loan expenditure^[1]may nevertheless enrich the community on balance. Pyramid-building, earthquakes, even wars may serve to increase wealth, if the education of our statesmen on the principles of the classical economics stands in the way of anything better.

It is curious how common sense, wriggling for an escape from absurd conclusions, has been apt to reach a preference for wholly 'wasteful' forms of loan expenditure rather than for partly wasteful forms, which, because they are not wholly wasteful, tend to be judged on strict 'business' principles. For example, unemployment relief financed by loans is more readily accepted than the fiinancing of improvements at a charge below the current rate of interest; whilst the form of digging holes in the ground known as gold-mining, which not only adds nothing whatever to the real wealth of  the world but involved the disutility of labour, is the most acceptable of all solutions.

(My emphasis)

It'd be nice if the battle were only against the right wingers, not half of the left on top of that — François in Paris