Eveofchaos
the explanation given at the time was those 8s judged the value of items inside to be too low for them to utilize so they let others attempt to complete them but I understand your point but if you reread them you'll see that great builders rooms emanated strength equal to rewards and the thiefs was considered impossible thief even said he cheated
I do hope the 7th rank or the immortal lands won't be all about inheritances ...It's kind of tiring that he's just been collecting inheritances and not really developing his own unique techniques ...gives the impression that nothing he's gained is really his own but modified from other's work it's almost like plot armor..giving the MC freebies