There are a couple things going on. Peoples' understanding of spoken English is notoriously poor. To get the message across in a high noise, high distraction environment like a jet, the spiels are longer and redundant so that you'll at least get part of it. Prefatory remarks like "At this time," are to get your attention so you're listening when they get to what they actually want. It's better than "Hey! Listen up!"

The other thing is that flat declaratives of what they want sounds rude: "Sit down. Put on your seat belt. Put your tray away. Sit up straight," makes them sound like your parents.