Short answer: It's what the biggest part of the audience wants to see. Longer answer: With the possible exception of PBS, television is a business. The networks and stations make their money on advertising. The more people that watch their shows, the more they can charge for ad space, so getting as many eyeballs as they can looking at their shows and ads is their number one priority. They spend millions of dollars figuring out what the majority (or largest minority) of people want to see and that's what they give them. Very little on TV happens by accident, certainly nothing that happens consistently, every day on all channels. The news anchors and commentators act the way they do because that's what attracts the most people to watch them, so their network can charge the most for ad space. It's all about money.