Because there are no non-debatable numbers for the real question here, which is: how much money does the government lose to middle class/poor tax deductions versus corporate tax deductions?
The reasons there are no reliable numbers:
--It's not always straightforward to determine what a multi-national earned overseas that perhaps, arguably, they should have to report to the US government.
--While the wealthiest two quintiles are said by the CBO to pay a higher effective tax rate than the others, those quintiles have more options to divert "earned income" into "companies" and other entities that act as tax shelters. You used to be able to divert almost your whole income this way (the infamous "$1/year CEO salary"), so for those individuals we need to look at their corporate tax burden to even begin to guess their actual effective tax rate as an individual. And even then, it wouldn't be apples to apples.
So it basically comes down to this:
You've got large numbers of people each getting an average of a few thousand in Federal tax breaks each year. You've got a smaller poor of wealthy people and companies, at least some of whom manage to pay effective tax rates as low as the lower income groups, and no way to determine precisely what they would have paid without those tax breaks being available. It's much easier to point fingers at the lower classes, whose taxation is well-documented because they can't take advantage of all these breaks and shelters. But the truth is, what the wealthy and corporations are costing us in tax breaks is much more difficult to estimate.