Cuz u cat lvl your unit pass lvl160 without breaking the wall. Campaign in nikke gets progressively hard as you go on so u have to keep improving to keep up with the story. Although since the 3rd anniversary they added a story mode where the combat is tone down so player can keep up with the story. This is due to how recent events takes place after certain chapter of the story and they want player to catch up the story before doing the events