I can make a few arguments.
A 2013 Federal Reserve study estimated that 55 percent of the value of estates larger than $100 million consists of capital gains that have never been taxed.
FRB: Finance and Economics Discussion Series: Screen Reader Version - Estate vs. Capital Gains Taxation: An Evaluation of Prospective Policies for Taxing Wealth at the Time of Death
Taxable estates generally only pay about one-sixth of their value in tax (a 17% average effective tax rate after the full exemption amount, other deductions, and loopholes). These estates aren’t exactly getting “double taxed” in most cases. If the capital is amassed via a 20% capital gains tax, then it is taxed again at a 17% average rate on the estate, the total tax burden is about the same as the top federal income tax rate of 39.6% that ALL high income workers pay.
Estate taxes encourage the creation of philanthropic avenues such as endowments and foundations to avoid estate taxation. In that way the wealthy dedicate funds according to their specific moral choices (education, health, fitness, parks, art, scientific research, whatever) rather than trusting to the government to make those choices for them.
All taxes distort the efficiency of markets, but estate taxes distort consumption and other economic activity less than other taxes. That minimized distortion makes it more efficient than other taxes