It doesn't just come down to which champ you picked. You're trying to make it like some simplistic thing that means X champ is op so pick X champ and win.
Rather, it comes down to the entire team you pick, and how that comp plays against the enemy team. It then comes down to who makes the plays to get their team in a position to utilize their drafted strategy.
It is not as simple as "oh this team drafted X Y and Z champ they automatically win". If you look at it like that then you probably don't have the understanding to watch ESports in general.
And as for the casters, they are working with the benefit of hindsight. It's so easy to say how great a team did in their draft when things go their way, but honestly, some teams are just straight outplayed. The casters are just like a normal sports broadcaster. They have great knowledge of the game, but they're not always right, they don't always know what they're talking about, and they're working with the benefit of hindsight.