Because it will maintain the Fedual system from generations before and the idea of a a person so big, we have to pay them..
The windsor family to continue the blood line. They are to be lavished with everything by the state.
They are also to present their genetics for analysis to ensure the correct blood line is maintained. This data to be made available to everyone as a "royalty stream".
Anyone with a "royalty streak" can claim they are part of the royal family and the rest of us as subjects, can park where they want and do anything they want, whenever they want, same as at the moment.