Why do all used book stores have cats? Don't tell me any reasons that explain how wonderful cats are. No. Cats are... well, they're a lot of things, but one thing for certain: they are not for me. People don't own cats, cats retain you for their staff.
I think at least half of all used book stores have cats. Stores without feline tenants are over compensated for as it is rare to find a catty bookstore without several creatures within. So, my conservative estimate is that there are at least 3 bookselling cats for every used and antiquarian book shop in North America. If you think you can escape cats in books about bookshops, well the ratio in fiction is at least 12:1! Now, this is all based on years of extensive research and a dislike of cats.
Before the bibliocommunity starts heating up the tar and plucking chickens, I offer these: 19th century trade cards featuring cats. These cats aren't too bad. They're mostly cute, they don't shed, don't make a racket or *ahem* mark territory.
Since nearly all of my readers (well, both of you) are "cat people" I post the following for your viewing pleasure. Of course, these are only a few contributions to the American Book Trade Index, which churns on. We are currently closing in on the 1800 image mark! Hooray!