If people choose not to spend, its their decision. Taking that money by force will lead to even less spending. So basically your solution is for the government to take all.
Only if you assume that all the money taken in tax goes into some black hole and disappears from the economy completely, which is doesn't; the taxes the government takes are partly used to pay off debt (which is a bit of a black hole) and partly to spend on infrastructure (creating jobs and getting people earning enough money to spend), on civil service/armed forces (creating jobs and getting people earning enough money to spend), on education (educating people so they can get jobs and earn enough money to spend), and on social services (so that people don't starve to death before they can get jobs and earn enough money to spend).
Taking the money that isn't being spent and spending it through the government is one way to ensure that the money isn't just stockpiled but gets back into general circulation.