What I really want to know is why do cartoon characters only have four.
As I said on another thread, four is easier to animate than five. Three would be even easier, but is too obviously false. (Five is about the highest number humans can intuitively grasp, and most people stop between three and four.)
It also depends on the cartoon character. I can’t believe I’m admitting that I know this, but the Teenage Mutant Ninja Turtles only had three digits per forelimb.
Well, yeah, but they weren’t exactly human were they? 
I’ve read interviews with a couple of famous american cartoonists who both said that 4 is the best blend of drawing ease and charater expressiveness… or something to that effect. For some reason, anime cartooists don’t seem to agree with that statement…
Here’s the thing. Sure, four are easier to draw than five, and with three, well, there’s a certain key expression that loses it’s pungency.
Imagine the Teenage Mutant Turtles flipping somebody off. It would look goofy. But if Bugs Bunny flipped you off, you’d know he meant business.
Not that I think that these delightful animated creatures would ever flip anyone off, even in, say, an animator’s sketch book not intended for public view. I’m just speculating here.
As opposed to, say, Bugs Bunny or Mickey Mouse.