We all know that MMO mobs have poor eyesight and hearing, or perhaps they simply don’t care when you kill their friends ten meters away. I realized the second part of the aggro radius question: a lack of object permanence.
Like small children, mobs lack the brain structure to know that something out of sight still exists. And they’re extremely nearsighted. If you can outrun them for five seconds, you are out of sight and out of mind.
Platformers frequently have enemies with perfect vision and absolutely no object permanence awareness. If they can see you, they will follow you endless and inerrantly. If you duck behind a waist-high block, they will go back to their standard patrol patterns. I also used to wonder about enemies who would shoot at your character with inerrant precision, only to have their shots blocked by walls or giant pipes. How did they even know you were there to shoot at you? Obviously they can see through objects, perhaps only able to see living creatures. And being able to see nothing except naked Marios and goombas is what fuels their rage to spit fireballs.