Similar rankings exist in pretty much every racing game. They assign each driver a skill level randomly at the beginning of the cup so the player doesn't win by getting 3rd in every race.

Good weapons are rarer when you're in higher places as a way of balancing it out. People who need good weapons to stand a chance, get them. People who are in first place don't get things that put them miles in the lead. It was like that in the older ones, too. When do you ever get a red shell in Mario Kart 64 when you're in the top two?