Why?
Because they already know how to be orators and can excel at that (I've seen both sides of this: there are pretty prominent examples of former debate champions now working in front-facing roles as PMs, VPs, etc in tech). Having mastered that, it's pretty unsatisfying to compete strictly at that level. Look no further than the intellectual depth of a debate with Tony Blair - an orator who can't follow his argument all the way down - for an example.
Here let me explain the logic of your criticism this way:
> These formatted baking pastry competitions because are useless! Really, who's to say whether any of these guys/gals knows how to poach an egg?