Tanks

Why are tanks so hard to beat in League? I had a game where I played Katarina and I was doing very well (11 and 1), and I was building magic pen items. So I started roaming to get my teammates going and it was all going well until later in the game. Anyways I was very fed and I could one shot everyone and then mid to late game came and I was legit utterly useless. Also Urgot is something else, but that's a different story. For starters, I don't understand the concept of making a champion that can build full health and armor and magic resistance and basically be unkillable whilst being able to almost one-shot anyone and anything unless you have an ADC like Vayne. But even with the true damage champions, one cc ability shuts them down easily. It seems as tanks are a free win and are very hard to play against; I think this should be more leveled out.