Poker is very similar to slam. The top players come on top very consistently, but it's not a guarantee. Just as the same team or individuals make multiple finals at national slams, the same players make final tables at the world series of poker. There is a lot of preparation, but a lot of luck as well.
I'm a professional poker player. I pay for my travel, airfare, food, entertainment, and enter tournaments with up to $10,000 entry fees. I am, without ego, among the top 5-10% of all poker players in the world. My preparation and study is extremely intense and academic.
That being said, I do not come in the top 5-10% of every poker tournament I enter.
Should I be given some sort of advantage to help dampen luck, and ensure greater success? Of course not. I understand that, in the long run, if I am the "better" player, I'll win the money. In fact, the element of luck is what makes poker exciting: if the best players always won, why would bad players come and play at all? In slam, if the same 10-20 people were guaranteed a victory, why would anyone else (who cared about winning) compete? That would be as boring for the winners as the losers.
Do people get robbed at poetry slams? Of course. That's part of the game. In the long run, the best competitors make finals and win trophies. Do not let your short-term, results-oriented examples contradict the fact that very often, very good writers and performers go very far in tournaments. The system isn't broken at all, and I say that as someone who would greatly benefit from pre-selected judges.