The only problem is that often that money has already been taxed.
So what? When I pay sales tax, I pay using money that's been subject to income tax. And whatever I buy may have been subject to an income tax/tariff on importation. And the retailer will have to pay tax on that money if it turns a profit. And the retailer's shareholders will have to pay taxes on any distribution from those profits.
I don't know why this "already been taxed" argument has gained so much traction with respect to the estate tax specifically, when it literally applies to every single tax on the planet.
It also oddly treats money like it isn't fungible, like there's something unique about a particular dollar where, once that dollar is taxed, it should not be subject to tax again. But the government doesn't tax dollars
, it taxes transactions
. And inheritance is, in its way, a transaction.