How can so many people, left and right of center agree on something yet not see any changes? That was rhetorical...the answer is because ultimately the government gets much of it's power via the tax code.
As JasonR says, there would be ways to help the poor but with a national sales tax, even the criminals and drug dealers would pay taxes. Not to equivicate Harvard with criminals but I heard today that Harvard doesn't pay a penny of tax including revenue on a $37 Billion endowment.
I am curious how donations to non-profits would be treated. The tax deduction is many people's motivation.