Smurfs

Why are smurfs allowed? They can often single handedly ruin an entire game for people and its completely fair and allowed. I thought the ranking system was in place for a reason, to test your skills and see how high you can climb within fairness, being slowly matched with people your own skill level. But it is completely stomped on by people making smurfs just to trash lower elo's either cause they've already been banned for toxic behavior on their mains, or they can't stomp those guys as easy. That is just as bad as making level 1 accounts just to stomp newbies. It is a toxic thing to do in itself and only leads to new people not wanting to try the game anymore, or people in ranked not wanting to play anymore cause they just get smurfs who destroy them and that normally leads to your team trashing you, my question is how is this fair?