A cat’s inability to convert plants into nutrients likely stems from their ancestral diet which consisted largely of birds and mice.
The cat’s prey offered pre-formed, active vitamins and fatty acids (i.e. vitamins the body can use just as is) and because their prey offered essentially ready-made nutrients, the cat’s physiology simply evolved as its needs changed. (See? Already ahead of humans and our useless appendixes.)
Cats, among other obligate carnivores, lost the ability to make certain amino acids and vitamins in their own bodies the way that omnivores and herbivores do, requiring they get certain vitamins and fatty acids from their diet instead. So, cats have a higher requirement for protein as well certain dietary sources of amino acids.
Cats are also less sensitive to the sensation of dehydration than other species and have a very weak thirst drive. This explains why you don’t ever seem to see your cat drinking out of that (rather pricy) water fountain.
The wild cat’s normal eating behavior may also play a role in water intake. Cats tend to be solitary hunters, hunting throughout the day usually catching and devouring from 6 to 10 small rodents or birds per day. A cat would get incremental amounts of moisture from their prey then, several times throughout the day, to nourish their body.
Little has changed for the domesticated kitty as far as dietary requirements are concerned. (A lot as far as the number of primo sleeping spaces Empress Puss n’ Cute gets to call her own.)
That’s why there are certain dietary requirements in pet food, specifically for cats, including amino acids like arginine and taurine. (Not so fun fact: Taurine wasn’t even a requirement in pet food till the 80s after a surge in feline DCM cases made it clear that it was necessary in a cat’s diet.)