In short, because we're feeding obligate carnivores - animals that derive no meaningful nutritional benefit from plant matter. That's why you'll not find things like kale or chard or other vegetables (or fruits, for that matter), in the recipe that's on my site.
There is an unfortunate tendency for many folks to assume that what's good for them is good for their cats - we all know how valuable whole vegetables and fruits are to a healthy human diet. But for cats? Not so much.
That said, there are plenty of ways to successfully prepare raw diets. I personally advocate keeping anything out of the diet that is not species-appropriate. That grows out of working with many people with cats suffering from inflammatory bowel disease; these cats are the feline equivalent to the 'canary in a coal mine' - the first to exhibit problems when ANY SINGLE THING that is not appropriate to their species is in the diet.
Some people add pumpkin, squash, and other vegetables as a fiber source. I suppose that's fine if your cat is okay with it, but there's no meaningful nutritional value for kitties from vegetables. My personal preference is to rely on psyllium as a non-carbohydrate fiber source if one is called for (I stopped using psyllium years ago). Keep the diet simple and as close to mother nature for cats as you can.
Caveat: I definitely don't have any monopoly on the only correct or acceptable way of feeding raw to cats!
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