The quick answer is because it’s easier and faster to draw four fingers than five fingers.
This worked well with cute cartoony characters and also had a big impact on output in traditional animation. Hanna Barbera productions, founded by William Hanna and Joseph Barbera , further streamlined the animation process by using cut outs which were reusable animated elements thus more minutes of animation was produced within a short time.
The four finger technique was started by Walt Disney when he produced the first ever animation with sound called ‘Steam Boat Willie’, featuring Mickey Mouse. However, the subsequent full length animated features like Snow White and the seven dwarfs had humans as main characters so Snow white had five fingers while the dwarfs maintained four fingers as they still very cartoony and exaggerated character design features.
Characters like Cinderella, Snow white and sleeping beauty were more labour intensive to animate.
Since hands are the hardest part of the body to draw, it’s acceptable to take some shortcuts.Besides, the story is usually so immersive that we don’t even notice that the characters have four Fingers!😉