That's a good question and one I've heard others ask as well. The only answer I have is...
Cats are fastidious groomers and spend up to one third of their waking lives cleaning themselves. You'll notice that they use a "face cloth" technique, wetting their paws and swiping them over the hard to reach places on the face, head, and ears. Cats will also groom each other. Mutual grooming is said to be a bonding experience, and is especially helpful in cleaning the hard to reach places like the head, ears, and face. You're part of the family, so you get a bath too, whether you need it or not.
