Actually, they don't pay for themselves. This is why Federal deficits have ballooned since the Reagan tax cuts. The Reagan philosophy was to starve the beast, in hopes that bigger deficits would lead eventually to spending cuts, as politicians woke up and realized that the country's finances were approaching the abyss. Instead, politicians of both parties simply got comfortable with gargantuan deficits, which is why our debt levels spiked, starting with the Reagan years and have never looked back:
What's particularly amazing is that we fought the Vietnam War and instituted the Great Society programs without bringing the debt up to present levels, as a % of GDP. Cutting taxes doesn't starve the beast (spending), because the beast is clearly morbidly obese. What it's done is run up ever-increasing charges on the national credit card.