Carbon only lasts a couple of weeks, maybe a month before it needs replacing. If you mixed it with matrix, when the carbon needed replacing you would have to remove all your media, seperate the matrix from the carbon and then mix in new carbon before putting it back in the filter. Seems like a lot of fiddly work. That's why they recommend putting it in a media bag. Take out the bag of carbon, dispose of the carbon, refill with carbon and replace the bag in your filter.
Carbon isnt something you need. It removes organic compounds. This is commonly tannins from driftwood, removing medications once the prescribed course has ended, and phenols which cause smells. Tannins eventually leach out, medication is only a temporary thing and if you keep up with your water maintenance you shouldn't get smells. Use carbon for what its intended for and when its no longer needed stop using it. Its expensive, and most of the time isnt performing a useful function.
