I would like to make the counterpoint: Why we don't need new monsters.
1. All the wondrous stories of the history and myths of our world were done with only humans and animals, and the creatures of myth and legend making only occasional appearances. You can attach personalities, backgrounds, cultures, legends to any creature in the book (even plain old humans) without having to create new creatures all the time. In fact, some of the best adventures I ran had nothing besides vermin, animals, and humans & a few demihumans.
2. Constantly added new monsters breaks verisimilitude, at least to me. Every time you add a new creature, especially a carnivorous one, you immediately ask your players to believe that this creature can support a reasonable breeding population, that they can find enough food, that somehow they are kept in check by environmental factors or other predators such that they don't dominate the world. I like my dungeons to have ecologies that make sense, and my campaign worlds to be much the same.
3. There are around 220 creatures in the Monster Manual (v3.5), and that's not including age variants of dragons. When you combine class levels and templates onto creatures, not to mention adding additional hit dice, you end up with tens of thousands of possible creatures. These creatures can act in many different ways depending on motivation and personality, and creatures with spell lists and spell-like abilities can take different styles in combat. You can take an Ogre with Cleric levels and Destruction and Death as domains and make them a melee juggernaut that opens up with a death touch and a smite and uses buffs to make them a big powerful melee tank. . .or they can be part of a squad of ogres and be the party healer, and bring in reenforcements with Animate Dead as his teammates fall, or they could be the shaman of a tribe that has a tenuous truce with the local human village, and that truce is endangered by mysterious raiders that he wants the PC's to track down and eventually the PCs have to stop an assassination attempt against him and fight alongside him.