Because not many people bother to do research into the subject and its origins, and it has already been adopted into American superstitions and pop-culture as that belief. If you have an issue with it, you should take it up with the people that tell their kids the lies you're frustrated with, as well as kids shows such as Arthur and the Fairly Odd Parents. They are the ones that promote these ideas to civilians at early stages in their lives, ergo raising them on that belief.
Mr. Elexion, Thank for your order of the Crazy Cat Lady Starter KitTM! Inside you will find your order of five (5) cats of the black variety. Also enclosed are instructions for proper care and feeding of your order. If you ever decide to expand your collection, feel free to contact us, toll free, at 1-888-CAT-LADY! We thank you for your patronage and hope to do business with you again!