The problem with inheritance tax is that produced very unfair results: the seriously wealthy don't pay tax on their estates: instead they 'donate' a painting or similar to the nation in lieu of the tax owed. And instead of it being carted off to the National Gallery, it's usually deemed culturally advantageous for it to remain where it is. The public have a theoretical right to go into these homes and view them. Do you remember Mark Thomas started knocking on the doors of country estates asking to view the publicly owned artwork.
Also, when parents of dependent children die, the estate would have been taxed at 40%, significantly reducing the pot left for the children's upbringing. That problem has been eased now by the ability to pass around £800k free of tax by being able to use both of a couple's tax allowance.
And very wealthy families help their children out during their lifetime anyway, so the unfairness exists, whether ou impose death duties or not.
The fact is, the people who paid IHT were the relatives of the middle classes, typical PAYE all their life types - not the extremely wealthy and privileged, nor those with enough wealth to make it worthwhile entering into tax planning arrangements. It was seen as a largely voluntary tax.
And to return to the OP's point: if you buy a house worth a million you pay a massive amount of stamp duty: it would be better to raise the amount you pay on purchase through stamp duty if you are serious about taxing people in large houses. Then is is genuinely voluntary, in that you don't have to buy such a house - to tax it on people's existing property is retrospective and unfair.