"If I knew a champion was nerfed in this modes, I wouldn't play them"
This is one of the reasons right here. Champions that are nerfed in these modes tend to be strong, not weak.
If you read "Ziggs does 10% less damage" the reaction is to not pick him because he's "bad". The reality is that he is still very powerful.
Understand the pain point in regards to wanting a source of truth, but showing nerfs/buffs in champ select could often mislead players who care about power into picking the opposite of what they're looking for.