It simply cannot be explained better than this; raising taxes slows growth, so you have to raise them enough to overcome the negative effects, at which point your net gain in revenues isn't worth nearly what it's costing. But the left is so intellectually bereft that they feel "raise tax rate, get more money to spend" is intelligent. Not because it makes any sense - it doesn't. Rather, they insist it does and so, to them, it does. Economics is what they say it is. http://www.willwilkinson.net/flybottle/2007/09/07/joe-sixpack-on-taxes/
And if liberals think health insurance is expensive now, wait until they find out how much it costs when it's free.
Obviously, at some point, this becomes true. However, look at the Excel spreadsheet of the GDP for the past 75-odd years posted here. What you'll find is that the GDP hasn't changed much over time. There are fat years, and there are lean years, but it's not as if the post-Aurthur Laffer years are generally better than the years when the government levied confiscatory taxes on the wealthy. The tax rates have changed remarkably, but the GDP hasn't changed much. I'm reminded of a joke about French academics, the punchline of which is, "Sure it works fine in practice, but how does it work in theory"? You could substitute "Aurthur Laffer" for "French academics."