2 Answers

Because we teach them to hate it.

There are plenty of cats that love water, according to Sandi Sawchuk, a clinical instructor at the School of Veterinary Medicine.

Big, wild cats, especially those that live in hot, arid areas, often love to swim. An Asian species known as the fishing cat uses webbed paws to dive for fish, frogs and crayfish.

Among domestic breeds, the Turkish Van is known for swimming. But most house cats shy away from water.

“Because cats groom themselves, we as owners tend not to introduce them to bathing like we do our dogs,” Sawchuk says. “Ask somebody who has show cats, which have to be bathed regularly and have been in the water since they were young. Those cats will tolerate it. There’s no fight at all.”

The spray-bottle method of controlling cat behavior — giving them a squirt between the eyes when they jump up on the counter — would make just about anyone wary.

“What’s the cat going to think about water then, when it’s only used for punishment?” says Sawchuk, who has three cats at home.

It’s not uncommon for the owners of Sawchuk’s feline patients to describe their cats sitting on the edge of the bathtub, batting at the stream of water from the faucet or shower head.

“When they have the ability to control how much they can get near the water and how much exposure they have, they don’t seem to mind at all,” Sawchuk says.

http://whyfiles.org/2009/why-do-cats-hate-water/

Cats know they can’t move with cat-like grace and speed in the water, I have seen panthers and lions  swim without prompting so it’s more likely how they were raised. I saw feral house cats swim in Florida and catch small fish under water.