Hard cheeses are dense enough that the mold can only grow on the surface. If you cut off the moldy parts and discard them, you’re getting rid of the vast majority of the mold. There will likely be some spores on the rest of the cheese, but not enough to harm you.

Soft cheeses are much less dense, meaning that the mold can penetrate below the surface more easily. If you can mold on top, it’s likely throughout the cheese and thus much less safe to eat.