The estate and gift tax is basically a flat tax and it keeps attorneys and cpas very busy with the IRS. A flat tax would increase the volume of similar issues that all taxpayers would use to avoid or reduce the flat tax.
oh for two......estate and gift taxes are only a tiny bit of accounting and attorney tax issues.....and to say that a simpler tax system (whatever it is) would INCREASE the avoidance costs compared to today is just a non starter.