As the game is currently set up, I agree with you. But, I don't feel that it should be this way.

I just think as the game progresses to pass the mundanes, they should be enhanced to keep up. Quadratic Fighters, as it were.

We're fighting flying incorporeal creatures intent on raining down magical attacks? Here are my options: Leap up and cut through their incorporealness, cut a hole in the wall for an escape route, focus on deflecting / reflecting their magical attacks, etc. That is flavorful, has an ancient folklore (Greek mythology, Epic of Gilgamesh) / anime precedent, and keeps the Fighter from becoming obsolete.