For some reason there’s been a lot of cartoons and comics over the years with human characters who only have 8 fingers and 8 toes. It’s been almost like a standard rule. The Flintstones, The Simpsons and others.
For the same reasons that they draw them with 4 fingers. It’s easier to draw 4 fingers and than it is to draw 5 and the style has a less cramped look. They want to continue the tradition of the earliest cartoons doing it, too.
