Why are there estate taxes after the owner passes away? Isn't it unfair for estates to be taxed after death? The person who bequeathed the estate probably purchased it with after tax money, so wouldn't it then be taxed twice? What is the purpose of the tax other than to generate funding for government spending?
From an equality of opportunity perspective, it is desirable to have some taxes on inheritance. This levels the playing field for each generation to some extent. If real estates are exempt from such a tax, individuals would prefer real estate over other assets, creating a distortion in the asset price market.

For a recent contribution to optimal inheritance taxation, see:
Piketty, Thomas, and Emmanuel Saez. "A theory of optimal inheritance taxation." Econometrica 81.5 (2013): 1851-1886.

For an overview over the equality of opportunity literature, see:
Corak, Miles. "Income inequality, equality of opportunity, and intergenerational mobility." The Journal of Economic Perspectives (2013): 79-102.