Because of clarity.
You see, cartoons originally were drawn by hand on paper then transposed onto a transparent sheet for animating.
One of the major drawbacks of that formative era was the dimension and quality of the equipment they were using. Those early pioneers quickly learned that intricate details were hard to replicate and capture and would often become incoherent when in motion. And that's when the projector was set up properly.
To ensure clarity, an exaggeration and simplification of the artwork was put into place. This lead to unique styles and expressive characters (and props) which captivated the audience who were unaccustomed to such things.
As with modern society, customers grew to expect a four digit hand and funny looking creatures in cartoons. This prompted Studios to keep the somewhat exaggerated look but were able to increase details as the technology advanced to keep up with customer expectations and what had become the standard. While five digit hands are not unknown in the later years of animation, they were mainly used for more serious films and characters such as Snow White and Johnny Quest. Pieces that were supposed to be more grounded using real world logic and story telling most times.
My favourite bit of Trivia is that Disney has (or had?) to pay Japan obscene amounts of money for them to leave the 4 fingers in because it's a rude gesture there. It's why Abe from Abe's Exodus got 3 fingers, because the studio couldn't afford to pay the Japanese as much to leave it at 4 fingers.