Inheritance tax is very unpopular, but it is also an economically damaging tax. Because it effectively taxes those who save instead of consume their income, it reduces investment and hence economic growth.
Inheritance tax is effectively an extremely high tax on one specific good: the welfare of your children. Since a lot of the stuff that buys welfare happens in the future, when they seek their own housing, cars, marriages, and children, this involves long-term saving and investment.
Long term saving and investment are what raise productivity and our standard of living. Inheritance tax tells us that we should direct more of societies productive capacity towards current consumption like holidays and less toward investing for the future. We get a short term gain at the expense of long term pain.