I don't think it's so much the yelling as it is the high pitched voices and EXTREME ENTHUSIASM attached to EVERY MUNDANE SENTENCE!!!! Actually, if you listen to little kids, they often talk just like this. I know my 5 yr old niece sounds just like one of those TV characters. She has a super high pitched voice and is always so happy and excited about what she wants to tell me that it comes out all loud and yell-y, even when it's NOT actually loud and yell-y. "AUNTIE K.!! GUESS WHAT, TODAY AT SCHOOL WE HAD !!!PIZZA!!! FOR LUNCH!!! AND GUESS WHAT, MY NEW BFF SAT NEXT TO ME ON THE SCHOOL BUS AND GUESS WHAT...." You get the picture. Exhausting for the person with functioning ears, but it's hard to squash their enthusiasm.
I guess some research somewhere must have shown that small children respond to high pitched, enthusiastic voices so those TV characters sound like that. Annoying, I agree.