Small nitpick: It isn't a "gift of $22 million" but a bequest. If gifts of 22 million were allowed free of tax, then absolutely nobody would ever pay inheritance taxes. The current annual limit for non-taxable gifts is $15,000 per recipient.
Wonder why wealthy US families like having lots of grandkids? Because each can receive 15K into their trust fund tax-free from each elderly relative per year. Amongst established families this is the primary means of moving money between generations. And if you are pregnant now, getting the baby born before Monday means a 15K bonus that it won't get if born on Tuesday.
Your reasoning about the allowances is backwards. If you lose a parent, an uncle and a grandparent all in one year, and get $22 million from each, then you have had a $66 million dollar "tax allowance". There's only one of you, but there are 3 deaths, so $22 million per death, ergo, a death tax. Conversely, if you work for 3 companies, you don't get to consider each income separately and pay only the bottom bracket of income tax for each of them; you pay the bracket that is appropriate for your total income, because it's an income tax, not a having-an-employee tax. (in the UK we also have a 'having-an-employee' tax, but that's handled separately).