I think that the question should be asked why is the Royal Family given the civil list.
The revenue from Crown Estates was removed from the Royal Family and at the turn of the century the government were giving the Queen just under 8 million a year.
In 2010 the coalition replaced the civil list with the Sovereign Support Grant which would be 15% of the Crown Estate revenue.
Being a Lancashire lad, I know that in 2011 the Duchy of Lancaster which is 44,000 acres was valued at £405 million which the government take and not the Queen. So the £30 million is a lot less than the 15% promised and that is only from that one estate.
Also included in her purse, was that she would pay for all her own transport (Which was paid for from the transport department). The household wages are also now her responsibility and that alone comes to 10 million a year.
Therefore the Royal Estates give a lot more back to the country than the government give to the Royal Family.