they dont have it becuase of the same reason as tribute.
Alot of games offer this kind of thing where you report during choosing, all of these games have had a issue where false reports were happening and the bot would just ban them, its unfair.
and riot knew with the current projects they cant do the report thing like PRACTICE MODE or a new skin idea.
anyways riot made reporting at the end of the game so that you would be able to understand them better, for example what if the toxic player has a family issue and apologizes for his words due to his(depression?).
and i know trolls and toxic champ selecters they are %%%gots.
anyways if you dont understand ill offer a simpler version of this.