Well, I'm no expert in Keynesian theory, but my understanding is that part of the goal is to dampen the boom and bust cycles. And governments issuing massive layoffs and service cuts into terrible economic times and already terrible private sector layoffs will (at least in the short term) make it that much worse as it produces that many more unemployeds -- and further damages the confidence and psyche of those still employed -- leading to a potential "ripple" effect of even more consumer pullbacks and more layoffs. In other words, when people need more jobs and more social services, that's the worst time for governments to slash jobs and slash services.
That's the theory as I understand it. (I'm not advocating that, just stating my understanding of the Keynesian "pump priming" idea.) Note that to have even a *chance* to be feasible long term, high deficit spending in a recession has to be countered by paying down the debt created by deficit spending -- and that means *slashing* spending in boom years. This is not something governments have ever shown themselves to be capable of doing.