Why did Namikaze Minato use ghouls to seal Kyubi no Youko's eternal life?

Brother, bijuu can't die! What's more, it is stronger than Kyubi no Youko (bijuu)

The most important thing is: that trick is not to seal up the ghost! ! ! Four generations are good at changing moves, and the ghost seal is to seal the soul in one's own body, and that move is in someone else's body! (This specific plot is not mentioned in the cartoon)