Because he doesn't understand that tax cuts increase federal revenue

Several conclusions follow from these data. First of all, reduction in high marginal tax rates can induce taxpayers to lessen their reliance on tax shelters and tax avoidance, and expose more of their income to taxation. The result in this case was a 51 percent increase in real tax payments by the top one percent. Meanwhile, the tax rate reduction reduced the tax payments of middle class and poor taxpayers. The net effect was a marked shift in the tax burden toward the top 1 percent amounting to about 10 percentage points. Lower top marginal tax rates had encouraged these taxpayers to generate more taxable income.

http://www.house.gov/jec/fiscal/tx-grwth/reagtxct/reagtxct.htm