Why aren't there more non-violent MMOs already?

Besides games like Puzzle Pirates and A Tale in the Desert, most MMOs have combat at their core.  This isn't because MMO players and designers love violence per se; rather, it's because combat provides a solid core to a game that gives it a ton of other mechanics.  Combat is an easy way to make a game "fun", for a number of reasons.  Here's a few of the mechanics that easily come from a combat-based game.

1.  Easily scalable challenge
Players like to gain power in a game.  In fact, in games like World of Warcraft, pretty much every action and goal in the game is done to make oneself more powerful in some way - richer, higher level, better equipped, etc.
Combat provides an easily scalable challenge for players.  Need a bigger challenge for a higher level player?  Just change the color of that Wild Bear, give it a few thousand more health and attack, and maybe double its size.

2.  Variety of attacks
Games have shown us that there's a lot of ways to kill something.  In a combat-based game, you can let characters be warriors, spellcasters, thieves, and all the other standard RPG jobs.  Your characters can fling fireballs, create earthquakes, shoot beams of divine retribution - you get the idea.
If, on the other hand, you make a game about mining for ore, or about growing crops, it's more difficult to come up with all these varied actions for a player.  Now, I'm not saying it's impossible.  For example, if I were to make a mining game, I could have some characters that use pickaxes to mine ore, some that dynamite their way through rock, some that reinforce the mineshafts with support beams, and some that refine the ore and supply the other players.  However, it would be a difficult task to create all these roles and make them each fun and relatively self-sufficient.

3.  Loot
Killing monsters and nefarious overlords doesn't just grant experience points, it grants loot.  Players love loot.  They really love loot.  World of Warcraft lets you find pieces of armor, big swords, magic rings and trinkets... each one makes your character look cooler, and each one makes you better at killing things.  A new sword that does 2.5 more damage than your last sword is clearly something to be happy about.
A non-combat game might not be able to pull that off as well.  If I make a farming game, how does a player get "loot"?  Do they sell their crops to someone who sells them better farm equipment?  Do I go mining and make my own tools?  Maybe I can grow magic plants that sprout tools instead of fruit?

4.  Gating
It's easy to restrict a player's travel by putting high level monsters all over the place.  If the land to the north is full of giants that can kill my player in one hit, I have to wait to explore it until I'm higher level.  This does two things:  It prevents me from quickly exploring the whole world and then getting bored with it, and it motivates me to keep playing (I want to get to a higher level so I can see what awesome stuff those giants are guarding).

5.  Easy source of conflict
Many definitions of "game" include conflict as a requisite.  Even in Solitaire (a decidedly non-violent game) you are, in a way, competing against the deck itself.  Having players fight against either NPC monsters or each other is an easy source of this conflict.  Making a non-combat game requires creating other sources of adversity for the player.  For example, SimCity players have to deal with disasters, traffic congestion, an ever-decreasing budget... etc.

As you can see, combat and violence are pretty much essential to the game mechanics in many modern MMOs.  A non-combat-based MMO would need to either replace these mechanics with other ones, or find a way to achieve these things in a non-violent way.  As I work on SpiritMUD, I'll post ideas I have about ways to get some of these mechanics in a non-combat game.