Because the raising taxes on just the top 100 would not pay off the debt is not a valid reason for allowing them the unfair economic advantage of paying a lower effective tax rate than those who make 1/5000th as much as they do.
Taxing just the Forbes Top 400 at the same average effective federal tax rate as those who make 1/1000th-1/2000th as much as they do would bring in about $25billion$ a year, or $250 billion over a decade. In the 30+ years that we have cut taxes for the very wealthy that would have been, adjusted for inflation, $750 billion. That's JUST having the .0001% pay a fair share. Raising the taxes of the TOP 250,000 or so (those making over $1million a year) to a fair effective rate, say 35%, would bring in about $200+ billion more a year. In the 30+ years that we have cut taxes for the wealthy that would have been, adjusted for inflation, $6 trillion.