Tax evasion added $3 trillion to the deficit over the past decade alone, an average of $300 billion a year, according to IRS data.
What comes to mind is how would they know this unless they caught everyone who was "cheating" at their taxes.
The Fair Tax is better than a VAT tax and replacing it seems to be really easy, the system already exists in almost every state. Don't even need to make a big deal about it, retailers already send in quarterly taxes so just add the sales tax to it.
The problem with a flat tax is it still taxes labor and hence many will force themselves to limit their labor to lower their tax.