That's true for a lot of animated shows. I support one of two theories, depending on what mood I'm in:
1. Animators are doing it to essentially cut corners because of how tedious animating is
2. Modern animators keep forgetting to add the fifth finger because they always saw cartoon characters with four fingers (basically muscle memory).
It took a while for me to get used to it. Then again, I've watched the MD pilot enough times to where I could do every line from the episode with little to no mistakes and audio cues being my only prompts.