From Lefi's testimonies, demonkins are mostly muscle-brained (enjoys fighting and sees strength as measure of status). Tribes like the horned beast, the maid, which is knowledge-oriented, are rare. Anyway, Dungeon Cores tend to pick up people from demon race (or create new demons with no previous life's memories), so in extension they're also mostly muscle-brained. Give dungeon's unique powers to muscle-brained people, and you can imagine they'll only grow more arrogant. Lefi claimed that she had destroyed plenty of dungeons herself, cause the Demon Lords thought too highly of themself. One that she remembered vividly was of a Demon Lord that intentionally challenged her, and that one was annihilated pretty badly.
I sure am personally interested in seeing how the other Demon Lords are faring however.
On a side note, I think people being brought from another world isn't a unique case that only happens to MC (since Lefi didn't appear to be surprised even after knowing MC's Demon Lord from Another World title). Though MC deduced that being reincarnated as a Demon Lord should be unique to himself, which is a fair deduction because MC's dungeon is much stronger than others (and thus being able to bring a soul from another world - Lefi also remarked that his Throne Room looked more majestic than the ones she had previously destroyed, likely due to it being stronger), and that may actually had something to do with Lefi. Dungeons are born in places with high concentration of mana, and with that region being Lefi's territory, I can only imagine her mana was also involved in making that region's mana stronger.