Because it is sport, quite an expensive one as it has to pay for all the bird food, pest control, land management and habitat conservation. Bonus there is a tasty wild crop of pheasant to eat.

Do tell of an equivalent better conservation system that is run? One that is self financing, on an equivalent scale, and as long term sustainable. RSPB efforts are tiny, and industrial farming doesn't do much. Its individual sportsmen that do most, plus a few who would do it anyway. Metropolitans do scant little but demand cheep food and a pub to go to as a tourist attraction when visiting the countryside.
I'm a huge fan of Sporting Estates be they one acre or 5000. Heck my garden I get both rabbit and pigeon to eat.