Interesting, after some research I find that a few very old motorcycles - Indians and Ariels - did have distributors. But these are a tiny minority as compared to all the Harleys and British and Japanese bikes in the world.

Also, I forgot that some old Harleys have the points in a housing sticking up above the crankcase. It's debatable if this should be called a distributor though, since the spark plug wires don't go anywhere near it.