I'm assuming it's because most people do not keep their cats indoors here. This is a very agricultural area. So quite a lot of cats are shop/barn cats. The cat I adopted in this state is actually tattooed, but I adopted him from a shelter a little over 200 miles away and things start becoming much more urban the further in that direction you go from my "city".

I think (without any research, just from vague memory) the ear clipping isn't actually all that painful to the cat, and the benefits of an immediate visual identifier outweigh it. I wouldn't even know where to look for a cat tat.