E.g. because on the other side of the ledger the government crowds out private borrowing (see my comment in this thread to byrneseyeview) and/or taxes others to get this money, plus this almost always is accompanied by attacks on businesses of all sorts, on Main Street as well as Wall Street. Extreme examples include 1937, when capital in the US went on strike and we had the Recession of 1937–1938 and didn't really recover until WWII changed the game.
In serious times of trouble, can you point out examples where this has worked in more than the short term? It didn't for the US in the Great Depression, and Japan is now nearing the end of it's second lost decade (we through they were pulling out of it, but that turned out to be a side effect of the world wide bubble that popped a little later this decade).