Why keep adding content when your current content isn't balanced?

Just saying, you obviously don't understand how rediculously well balanced the game is right now. Aside from a FEW flukes, everything has moderately simple and easy counterplay. However, to answer what you're saying, I think it can only be described appropriately in a list.

1. The balance and design teams are COMPLETELY different teams.
2. The reason champions get "OP" at times are because of meta-shifts, which are easily summarized as the changing of what is overpowered and what is underpowered due to the widespread learning of how to counter something that was before considered op. AKA everyone learning to play against something.
3. To go with the Meta shift thing, certain things just become popular because of random things like blitz becoming popular because he's funny or teemo becoming a top pick because he's easy. The design team can NOT do anything about that nor can the balance team.
4. Every action has an equal and opposite reaction. To buff Feral Flare is to buff WW which is also to nerf sejuani because now she is even farther below WW.
5. Adding a new champion is a GREAT way to nerf a pre-existing one. For example adding gnar into the game when jax at the time was one of the mnost overpowered champions maybe even the MOST overpowered champion was a GREAT way to nerf him, because without ruining him and making him COMPLETELY unplayable, they made it so there was viable counterplay.

IDK man its late at night im not thinking at 100%, but i hope this is able to give you an idea of why the system is how it is.

Real quick: There are different teams working on different content and projects. When they are done, they basically just go tell their bosses that they are done and boom it hits the rift. Seems pretty simple to me.