Why are players allowed to play champs they have no experience on in ranked?

I never understood riots take on ranked as a whole, players are allowed to get away with many things because they have lots of freedoms they shouldn't have in a competitive environment. One of these is the fact that players are allowed to literally first time a champion or play a champion they have very little experience on in ranked if they want to. Why are they allowed to do this? Isn't this supposed to be the competitive game mode of League of Legends? Why are you allowing players to pick champs they have no experience on only to ruin the game for everyone?
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