Simply put, the tournament is just too unpredictable. This is part of what makes it so exciting. Outside of a 16-seed taking down a 1-seed, which has only happened once, anyone can beat anyone. If you flipped a coin to make your selections, the odds of a perfect bracket are 1 in 9,223,372,036,854,775,808. This is because there are so many games in the tournament. Your true odds are slightly better than that since not all games in the first-round are true 50-50 splits.