Questions regarding champion balancing.

So, I am pretty decently new to the game (lv 25), and I have been seeing the same things over and over again every time I go into draft pick.

Team 1's bans: LeBlanc Cho' Gath Zed

Team 2's bans: Katarina Sejuanni Irelia

(insert Teemo, Gragas and Nidalee in here occasionally) My question here is, why? Why are these champion always getting banned? I haven't been in a game where at least 3 of these champions haven't been banned. I am wondering why these champions are always being banned and why no one is dealing with it.

If I where to go on Youtube and look up 'Most OP champs in LoL', LB, Kat and Irelia would pop up for sure. So why aren't they being changed?

But who knows, I'm new to the game and am probably wrong/under-educated on this topic.

Tyler, (Ps. I love Blitzcrank :p)