@Fnl
Because their dumb asses went at rocks, steel and iron, with swords. Got frustrated they were not getting an immediate effect, and gave up too quickly is my guess. Seems like all the training did was just get them to just keep doing it until they finally are able to brute force it by slowly and literally chipping away at it.
Maybe this isn't how that world works, but if I swing a sword at solid metal or a rock, the sword gets chipped and dulled way faster than that rock or metal gets destroyed. And depending on the material's hardness, their swords may not even be able to scratch it.
We can assume that their hightened physical abilities can bridge the gap to an extent, but maybe that day of training really did a lot for then too. Idk. Levels seem to work oddly and inconsistent in this world. Almost as if the story can't decide if skills or levels matter most in determining one's ability, so they just mixed and matched them here and there until there is no power scale to speak of besides MC being clearly better, but who knows by how much exactly?