Unlike omnivores, cats are unable to synthesize arginine, taurine, methionine and cystine, arachidonic acid, niacin, pyridoxine, vitamin A and vitamin D from their own organs and must get it from other sources. Their livers and kidneys simply cannot make this material from other materials. For the most part this list of nutrients is not available in complete form in plants.
Our bodies for example make vitamin D from sunlight via our skin (d7). But can also get it in multiple base forms and synthesize it from animal based foods containing d3 or from compounds containing D2. Cats however only have the ability to use D3 and cannot synthesize D7 or convert D2 to D3 (omnivore liver)
In theory you could make food in a lab that is technically vegan and supplies the above nutrients. Nobody has done this.
I’m always unsettled when discussing this topic that people can readily accept and understand that pets are unable to digest the same foods as us when it comes to toxic things like grapes or chocolate. Their livers and kidneys can’t break those compounds like caffeine and tartaric acid down as an efficiently as ours.
Similarly people readily accept the idea that a bird can eat nightshade and a deer can eat poison ivy because their bodies can digest foods ours can’t.
But that somehow doesn’t help them infer the same thing can mean those animals cannot get their nutrients from foods the same ways that we can and vice versa. That human dietary concepts don’t just magically apply to the whole animal kingdom