Quran 5:3-5
if you find the hunted bird or animal alive, you need to do Tazkia by slaughtering its throat to make it Halal for you, and if you find it dead, it is declared Halal, no further tazkia is required.
Well one can’t expect the blood to be completely drained off from the body if the it has been shot randomly anywhere. Slitting the throat mean the main vein has been slit open (the biggest vein) which serves as an outlet for the blood to be pushed out of the body. When the animal is already dead, there’s no point of slitting the throat because the heart has stopped pumping. Indeed there’s a hikmat behind every hukum of Allah.