That's because the mega wealthy use private foundations to shield unrealized gains from any sort of taxation, whether income or transfer taxes. Cases in point: Bill Gates and Warren Buffett.
The big argument that those on the left make for the estate tax is that it adds progressivity to the tax system. I dispute that as a legitimate goal, but assuming it is, Riordan suggests a far more effective approach to reaching it:
All inter vivos gifts and transfers at death of appreciated assets should be treated as income tax realization events. Income taxes should be paid. All personal expenditures like charitable deductions and residential mortgage interest should not be deductible in computing income tax liability. Personal expenditures should be made after tax.
I most especially agree with that last sentiment. If the charitable deduction for income and transfer taxes isn't repealed, it ought at least to be curtailed. Note that Riordan isn't calling for a gift or bequest to be counted as income (alas, that's what I thought on my first quick reading) but rather, that the capital gains be recognized and taxed at the moment of transfer, presumably paid by the donor or the estate.
Any chance that this Congress could do something this sensible? No, it would offend far too many entrenched interests.