But he didn't, so they aren't exactly idiots are they?
Besides, if they had hidden them someplace impossible, then we wouldn't have a story, and a great concept like the Infinity Gems would have been taken off the board.
I'm just pointing out the flaw that the Hood is currently exploiting. The Illuminati are functionally morons and spectacularly failed. If the Hood is able to run around snatching these things with relative easy, just think what ANY bigger, cosmic threat would have done.
Giant fish? Danger Room which requires you to fight a bunch of lame-ass X-characters? Two (apperantly easy to break into) super vaults? I'm surprised Dr. Strange didn't give his gem to Night Nurse as a necklace to wear. (Although that could happen next issue).