My headcanon is that Celesteela and Kartana have a mating system that's a sort combination between bedbugs and deep sea angler fish. Like deep sea angler fish, the male is much smaller than the female and it's only purpose is to mate. This is why Kartana has such a pathetic mouth and seemingly no digestive system. Everything about its tiny body has degenerated into being an effective penis. Like bed bugs, Kartanas fertilize Celesteelas by stabbing them. It is said in pokedex that Kartana was observed cutting down a steel tower. He wasn't cutting it down for no reason. He was confused and horny, attempting to mate with what he thought was a Celesteela. Kartana has such high attack to allow it to penetrate Celesteela's high defenses and thick metal body, which have co-evolved in a sexual arms race.