clearly you don't know the history of cycling, WC has always been something special, for all the riders but also for the cycling world in general. Vuelta is nowhere close to WC in terms of prestige. After the Tour, WC is the most prestigious race a cyclist can win, this is what the history of cycling says. In the past the Giro was more prestigious, I am talking 40-50 years ago, but over the decades it has lost quite a lot. So right now WC is ahead also of the Giro.
"while in WCs, we can have a random rider win them": what exactly do you mean by that? to win a wc you have to do something special, clearly it can happen that in some years there is a less strong rider winning a WC but that's true also for the Vuelta (just look at the winners in the two races in the last decades)