[lbriner]

In the UK (and I'm sure elsewhere), inheritance tax is the main vehicle to try and avoid dynasties being created once one person has accumulated massive wealth.

As others have said, it is income that is taxed, there will be plenty of people who own capital (e.g. inherited a large country house) and who don't have the cash to pay a tax on it and presumably it would seem unfair to force them to sell it (and hope they get decent money for it) in order to pay a tax on it.