Now MC can freely talk with the guild leader of this entire place so he could easily claim his right directly but meh, it's manga.
But there's also a certain degree of Darwinism at play here. That being: How long until this idiot gets himself into a situation that he cannot survive? And all because he's been taking so much credit for himself that he got ranked up without actually having the ability to maintain that ranking.
There's a manga I've been following - gone blank on the name of it - but at some point one of the higher beings pulls the MC aside and reveals to them the entire reason for the dungeon system to exist.
And that was simply to keep the power-hungry humans in check.
It's beautiful in its simplicity. Humans who have too much greed or ambition usually wind up dying in the dungeons, while those that are more humble and treat dungeon diving as an occupation rather than a ticket to wealth, fame, or glory... they may not become legends, but they do prosper in their small way.
Even the "Heroes" who do become legendary adventurers have their place in the system, as their exploits goad and entice others in trying to attain that same level of success. As you'd expect, very few of these people survive their own hubris.
And I fully expect to see the same here.