Why do so many tributes die at the beginning?

It's disappointing not being able to meet half the people in the hunger games because they all died at the beginning. I was really impressed by Thresh and Rue and I even decided the landmine planting dude's name should be Ash. But why kill half of them at the beginning? You'd think they, after being forced to watch the Hunger Games would run away at the start like Peeta. Is it simply lazy writing that a bunch of potential interesting characters just end up dead before we really meet them?