In a normal conversation, some people will talk more than others. This is the norm. People come with varying degrees of interest, extroversion, and desire to dominate the conversation.
But in TV, your characters come with perfectly matched degrees of extroversion, perfectly matched understanding of who gets to talk and when, and a perfect understanding of when someone is done talking. The division of conversational airtime between five people is a perfect 20/20/20/20/20.
This trope is a subtype of unrealistic diction and more common in sitcoms and cartoons, but less so in movies and written fiction. It's especially common in children's television, perhaps in attempt to align with the simplistic view of conversation children have — everyone shares fairly, everyone gets their equal turn, and so on. That's what they're taught, so the fiction makes more sense for them if it's portrayed that way. It can also be a consequence of the Law of Conservation of Detail. Why bother to have that character in the room if s/he's going to be quiet most of the time (and also isn't The Quiet One)?