The short version
Most pet cats share three things: an ancestor built for dry terrain, a coat that only insulates while it stays dry, and a general discomfort with situations they cannot easily move through or predict. Combined, those three factors produce a default aversion strong enough to look like a species-wide rule, though individual cats and even entire breeds fall well outside it.
A cat pawing at a dripping faucet and a cat bolting from a bathtub are the same animal responding to two very different situations. That gap between them, more than any single trait on its own, is what actually explains the stereotype.
An ancestor that never needed to swim
Domestic cats descend from the African wildcat, Felis lybica, a species that Wikipedia's summary of the taxon notes inhabits deserts, savannas, shrublands, and grasslands across Africa and West and Central Asia, with some populations domesticated roughly 10,000 years ago in the Near East. Nothing about that range required regular contact with open water. The wildcat's diet leaned on rodents and other land prey, and a cat that hunts on dry ground has no evolutionary pressure to develop comfort with swimming or a coat built to shed heavy water, the way species that forage in or near water do.
That inherited baseline explains much of the stereotype, though selective breeding since then has been comparatively light on cats next to dogs, so most of what a housecat does around water still traces back to what worked, or did not need to work, for a wildcat on the savanna. Geneticists place the domestic cat among eight principal lineages making up the modern cat family, and terrain varied widely across the other seven; the fishing cat, for one, developed partially webbed front paws built for hunting at the water's edge, a path Felis lybica's desert-and-savanna ancestors never needed to take.
The arid legacy runs deeper than behavior. Per the same Scientific American reporting, Siracusa notes that many housecats, especially those fed dry food, need active encouragement to drink enough water, because over evolutionary time cats came to get much of their hydration from prey rather than from standing water in the dry landscapes where they evolved. He recommends owners set out several water bowls, including at least one away from the feeding area, to work around that inherited habit.
What actually happens when cat fur gets wet
According to the Merck Veterinary Manual's overview of feline skin structure, a cat's coat helps regulate body temperature by trapping air between the secondary hairs, and that function "requires that the hairs be dry and waterproof." Soak the coat through and the insulating air layer collapses, leaving wet fur that clings to the body, adds weight, and slows the cat down instead of protecting it from the cold.
Carlo Siracusa, a veterinary behaviorist at the University of Pennsylvania, made the same point directly to Scientific American: wet fur can leave a cat more exposed to cold because, like most furry mammals, cats rely on that fur for insulation. Wailani Sung, a veterinary behaviorist at Joybound People & Pets, adds a sensory angle: cats have whiskerlike hairs across much of the body that help them read their surroundings, and she speculates that getting those hairs wet might disrupt how a cat perceives the environment around it.
Losing control matters more than getting wet
Siracusa put the deeper mechanism in plain terms to Scientific American: "Cats in general are averse to new stimuli that they feel they cannot control." Full submersion checks several boxes at once for a cat: it restricts movement, it is unfamiliar to most housecats, and it happens on a timeline the cat does not set. Sung pushes back on the underlying stereotype itself, telling the magazine that painting every cat with the same water-hating brush overlooks how much individual temperament varies from one cat to the next.
Sung also points to a simpler explanation for why cats seem more water-averse than dogs: exposure. Pet dogs are more often taken outside, bathed regularly, and given repeated low-stakes chances to get used to water. Indoor cats mostly are not, and a cat that grooms itself with its tongue has little occasion to encounter water at all until a bath forces the issue.
The cats that don't fit the pattern
The clearest counterexample is a documented breed, not an anecdote. Per the Cat Fanciers' Association's own breed profile, the Turkish Van, established in Britain in 1955 from a pair of cats brought back from Turkey and recognized by CFA in 1988, is listed with the personality trait "enjoy water," and its breed standard specifies a semi-long, cashmere-textured coat that is "soft to the roots with no trace of undercoat." Without the dense undercoat other cats rely on for insulation, wet fur is less of a liability for a Van, and CFA's history of the breed credits founding breeders with observing kittens voluntarily playing and swimming in pools or lakes.
CFA describes two other breeds in similar terms. Bengals, per the CFA breed profile, "are fascinated by water and may enjoy watching you brush your teeth, or even try to join you in the shower," a trait sometimes linked to the breed's ancestry from the Asian leopard cat, a wild felid native to water-adjacent habitat in South and Southeast Asia; CFA accepted Bengals for registration in 2016. Maine Coons get an almost identical description: CFA calls the breed "fond of water," noting that many will watch a dripping faucet in fascination, wash food or toys, or try to join an owner in the shower.
Why the same cat plays in a dripping faucet and bolts from the tub
The faucet and the bathtub put a cat in almost opposite positions. At the faucet, only the paws get wet, the cat controls the interaction, and it can walk away at any point. Moving water also taps into a hunting instinct: a cat batting at drips is reacting to motion much the way it would react to a small, erratic prey animal. In a bathtub, the same cat is fully wet, weighed down, and often physically restrained, with none of the control that made the faucet tolerable in the first place.
That contrast is the actual mechanism behind a stereotype that does not hold for every cat: a specific bundle of conditions, wet, heavy, cold, and out of the cat's control, that a bath reliably produces and a dripping tap almost never does.