Depending on local laws, income and other assets obtained by a benefactor within a certain amount of time (perhaps up to a decade) before death may be subject to inheritance tax or higher inheritance tax rates before being passed to beneficiaries.

And if the parents don’t declare it, then the beneficiary will have the same “bag of cash” problem but a few years down the line, because the government has no idea where it came from.

Edit: Put a benefactor where I should have put beneficiary.