Implausible Deniability
Any player that has spent any time dungeon crawling is familiar with the scenario in today’s comic. They walk into a room, violently and loudly slaughter every living thing in said room, remove all valuables from the cooling corpses, and then move onto the next room whose inhabitants seem blissfully unaware that their coworkers were just bloodedly murdered not twenty feet away. The alternative – having the monsters in the next room charge in to help their allies – seems unfair to the players: triggering two encounters at once.
It gets worse: the 4e starter adventure included in the DMG has a tough fight the characters will need a short rest to recover from. The next fight is in a room separated only by a small gate — not even a proper door!