Actually, they have. Seriously, read.
BTW:
Critics say that this amounts to increasing taxes at a time of high unemployment, and that instead the tax cuts should be extended as a stimulus measure. This overlooks the fact that tax cuts are an inefficient form of stimulus, because many people choose to save their additional income instead of spending it.
If the goal is to encourage growth and employment immediately, it would be better to let the tax cuts expire and dedicate some of the increased revenue to real stimulus programs. Alternatively, if some tax cuts are extended — as it seems likely that at least those for the middle class will be — there should be provisions to eliminate them automatically when unemployment falls to a preset level.
The Bush Tax Cuts and Fiscal Responsibility - NYTimes.com
But economic research suggests that tax cuts, though difficult for politicians to resist in election season, have limited ability to bolster the flagging economy because they are essentially a supply-side remedy for a problem caused by lack of demand.
http://www.nytimes.com/2010/09/11/business/economy/11tax.html
The truth is that there is virtually no evidence in support of the Bush tax cuts as an economic elixir. To the extent that they had any positive effect on growth, it was very, very modest. Their main effect was simply to reduce the government’s revenue, thereby increasing the budget deficit, which all Republicans claim to abhor.
Bush Tax Cuts Had Little Positive Impact on the Economy - TheFiscalTimes.com