merrymouse - Because they are fundamentally different things.
Inheritance tax is basically like saying if you give a gift to someone, then you have to give money to the state in order to receive that gift.
You OWN your possessions. There aren't many things that once you buy you are required to pay continued taxation on. If I buy an engagement ring, I don't have to keep paying tax on it - after the transaction it's mine. If I want to then give it to someone, that is a private matter, and the state has no business in it.
It is absolutely ridiculous that people who have worked their arses off, and have managed to get a bit of wealth for themselves should be penalised for it at the end of their lives. And even if they aren't the ones paying the tax on it, they are the ones who have worked to earn these things so they can pass them on one day. It's like a massive fuck you.
That isn't to say that if people work hard they will always luck out, but when they do, why the fuck should anyone have the right to say "oh, sorry your parents are dead. So we'll just be taking our 40% of their life time accumulation of wealth which they expressly want to leave to you in their will".
The tax rate starts at 325000 for fuck's sake. Sure, in some parts of the country that's a lot of money, but even where I grew up in the middle of absolute NOWHERE current house prices are that or above.
It's basically society punishing people for having the gall to do well in life.