List of reasons:
1. A first timer will most likely lose the lane AND huge chance of losing the game
2. Will make teammates frustrated and mad which will result in chat toxicity, which will result in chat restrictions AND banned players, all because little Jimmy felt like playing Katarina for the first time in ranked...
3. A first timer will not even learn to use the champion properly getting killed over and over again anyway so will just end up going to normals in the end if player liked the champion.
4. If all players know at least the basics of a champion to enter ranked with that champion then toxicity will drop and also LP loses will be prevented, have minimum mastery required to be able to play a champion in ranked be level 2 which can be reached in just 3 or 4 games in normals.
5. As of right now I can easily go into a ranked game and first time pick zoe in ranked and go 0-10, make 4 people lose the game and report them for being toxic to me because I fed, and who knows maybe a player will get banned all because I decided to first timed zoe...its that bad.