A person understands the reason they're eating only plants with no animal byproducts. An animal doesn't. An animal is just confused and irritated that their food was changed for the worse by their owner. If their diet and mountain of supplement pills/powders did not actually meet their dietary needs because it wasn't an exact match for their regular food or natural prey, they would still end up malnourished. And not every cat's dietary needs are the same or stay the same as they age.

Malnourished or not, you also wouldn't be able to stop your cat from finding a mouse or insect which snuck into your home and devouring them to enhance their compromised diet. You cannot make a carnivore vegan, you can only abuse them into living in a way they do not naturally live and do not want to live, until they find a way to avoid you for just long enough to go against your wishes and savage another animal, as is their instinctive nature.

Furthermore, do you really think animals have no joy in what they eat, that that's only a human quality? Nutrition doesn't matter to the animal, they just want to eat what they want to eat. Cats almost never turn down an offer of cream or milk despite 90% or more of them being lactose intolerant. It's not nutrients their body needs or can absorb, and actively makes them feel ill. But they want it anyway because it's tasty and they aren't able to consider the consequences of their actions as far in advance as humans can.

Edit: In fact, going off that same point but for humans, you could probably make a human live off some kind of tasteless nutrient bar that gives everything you could need, but it wouldn't mean they'd enjoy it. Oh wait, we did do that before, as a cruel punishment for prisoners in the US, fucking nutraloaf!