First, the appeal of Superman is his heart more than his strength. There's one comic where he fights a giant robot and stops a runaway train, but the scene everyone remembers is when he talked someone down from the edge of a building.
Second, Superman may be invincible, but Lois Lane isn't. It's easy to defeat a villain, but much harder to defeat them while also keeping Lois safe. And she actively invites danger, so it's always tricky keeping her safe.
Third, not every problem can be punched. Luthor's greatest weapon against Superman isn't kryptonite; it's Public Relations. You can punch a monster, but that won't help you stop a smear campaign.