Master Yi has a 42% win rate

For a couple of reasons. First, he is still frustrating to play against. If he ever snowballs, he is unstoppable. Also, he is almost impossible to outduel.

Second, people take a while to stop banning things. It took weeks for people to stop banning Mundo even after he became actually balanced.

Third, people still have nightmares of tanky Master Yi with Guinsoo/Titanic Hydra and Sated Devourer. I can't blame them.

Most people dont actually think of what is strong when they ban they just ban things they were told is strong. Zed was still being banned when he dropped to a 42% winrate so I am not shocked it's happening with yi. That's why banrates don't tell riot much about strength because banrates have little to do with strength most of the time.

To not let your own teammate to pick him

A fed With 2 fire drakes and and an ocean drake is balanced :^)