In many games, believe it or not, is a actually intended. Once certain mobs start dropping certain items, people start camping only those mobs and ignoring everything else causing a large bottle neck for the population (in the case of multiplayer only obviously).
Additionally, say you give a mob 0.01% chance to drop The Sword of a Thousand Truths. And of course everyone wants the The Sword of a Thousand Truths. So every day you go out and farm these kobolds until your fingers are numb rather than exploring the world.
Finally, by providing a clear path to getting what you want, you will likely max out sooner and stop playing until the next content patch, which in turn means less revenue for your game