The AI does have creatures and npcs wander around aimlessly... or apparently so to us foolish players.
The original intent was that every creature and npc would have dislikes, desires, affinities and needs. They would attempt to find/get those desires and needs. So, for instance, a goat wants to eat... anything I guess, so it looks for food. It likes other goats. It dislikes wolves. It likes humans that are "good", but not those that are "bad".
So, it wanders around at random, looking for those situations and trying to find them. From our perspective, however, we can't always see them doing this in an obvious way. BUT, if you sit and watch a large group of animals or creatures, you can see them stop a bit, almost like they are pathfinding for something. Then then turn and move off. Sometimes towards something in particular, othertimes just at random.
Yes, they turned off most of the original True-life AI they attempted (I seem to recall a dev telling us at some point that the players in Beta managed to kill off entire populations of deer, so they had to turn off the 'reproducing' parts when whole areas lost their animals. )
However I am pretty sure the behaviors for affinities and dislikes are still there for many things.
As for npcs and creatures 'stopping up' recall sites -- sometimes the problem is that they are trying to go in direction X, which is through your house (or wall or whatever.) They have no reason to change course, so they keep trying until something else draws their attention. It looks like the 'find another interesting path' code only attempts it every 5-10 seconds when they get to a stopping place, and sometimes that means they just sit there.
I've watched the terathans and ophidians fight, the only reason they wouldn't is if the paths didn't come close enough for them to notice each other.
The other problem is that in the first few years they added a sort of dampening mechanism to spawns. If the spawn isn't "bothered" by anything for more than (I think) 5 minutes, it despawns and a new spawn is triggered. They did this because spawns were clogging up the server -- they tried to keep certain spawns rare or uncommon, which meant if the creature/npc/animal never went away or never got taken from the subserver, it wouldn't spawn again. We see that effect with taming and certain higher level creatures.
I have to say, I do miss being able to gate dungeon creatures to the surface. While it could cause some problems with newbies, it would have allowed us to do some more interesting player events.
Anyway, um... Ok, if we're going philosophical:
We have all sorts of undead creatures -- why are they pretty much only undead Humans? Why aren't there undead Orcs, undead Trolls, undead Elves, undead deer?
Why hasn't the Elf world tree fallen down from the weight of the millions of step stools, wind chimes, flower garlands, and broadswords we've dumped on them?
Why do many finished products weigh more than their component parts?