Why are new champions picked in high elo the day they come out?

Do experienced players not need to practice new champs or are these players just trolling? Additionally, why do so many players feel the urge to instalock new champions and even play them in inappropriate positions (Kayn Top, Kayn Mid, even Kayn ADC) even though most of them haven't played all other champions in that position yet (if you're a new player, both Kayn and Warwick would be new to you, so why would you prefer Kayn to Warwick)?