>>280206121 (OP)
Because there is no point for the story if the villain got defeat by someone else. THe entire story is built upon the connection between the MC and the villains, how they oppose each other, why they oppose each other, what are they believe in,.... Having anyone else defeat the villain basically flush all that development down the drain
Let look at the arc shall we? Marik is built up to be a parallel to Yugi, both the owners of the millennium items but the difference is their relationship with their alterego. Joey defeat Marik doesn't help solve that issue. Not only that, the entire arc was building up the fight with 3 gods cards, Joey ended up winning mean that not going to happen because Joey can't use Ra (it has been established pretty heavy that no one beside people who know Ra true power can use it)
