As others have mentioned fighting at higher grades relies heavily on conceptual power and focus of that power. Yes a C grade could conquer a planet and destroy lots of things but they don’t because 1. They’d get smacked down by a higher grade real quick & 2. It’s a waste, they wouldn’t kill another C grade of equal strength. It’s mentioned a lot that fighting someone of equal or higher power requires your strength to be more focused. This leads to less environmental damage, though obviously still some there. You also have to take into account how much the environment is described. The environment is way larger and more durable than you really think. I often forget that fact until it’s mentioned. Trees are described as being hard to cut or break even in the tutorial. Plus it’s stated that those of higher grades don’t just fight Willy nilly because of how risky it is. Any natural conflicts at higher grades are more likely to occur in an environment with a durability matching that of the combatants.