If you were able to track a particular sum of money (yes, I realize money is fungible so this would be ridiculous), you would see the same money being taxed dozens of times. Somebody gets paid for work. Taxed. He uses that money to pay his housekeeper. Taxed. She uses the money to buy something. Taxed. The store owner wins money with it at the casino. Taxed.
The way we tax is transactional. When certain transactions happen, the government taxes them. When other transactions occur, the government doesn't tax them.
You can make coherent arguments that an inheritance is not the type of transaction we should tax. There's nothing magical about, say, earning income that makes that a taxable transaction-- we just decided that was a good transaction to tax.
But it doesn't really make sense to say that a particular transaction shouldn't be taxed because the money in that transaction was previously used in a different transaction that was taxed. Because that's true of any tax.