Hey CSL.
Another thought provoking thread. I'm just going to respond to your initial post so sorry if I go over stuff that's already covered by other respondants.
Speaking as an inveterate world builder the questions you've asked are important ones to me. But I'd disagree with the answers you've come up with.
to discuss a couple of your specific examples first:
If important natural resources are guarded by Gobbos (et. al.) either people will trade for them or take them by force. Or, conversely, the Gobbos with their monopoly on important natural resources will come to dominate the humans (economically, militarily, culturally). And if there can be cross-cultural information exchange between Dwarves and humans why not Gobbos and humans? It won't be all war just because Goblins are supposed to be evil. Least ways not in my games.
This would be a good time to segueway into racial determinism in RPGs. I'll try to be brief. As has been pointed out this idea of racial determinism has its roots in good old fashioned 19thC racism. It continues into modern fantasy through the likes of LotR and Conan.* I don't use racial determinism in my games much. Oh the basic tropes are there: Orcs are big, rather violent and uncivilised but they're not necessarily evil or opposed to humanity. But I've gone on at great (read: interminable) length on this topic in other threads and will spare folks from it this time.
But I'm getting side tracked off your main point: a game world not changing.
It's a pretty common trope in fantasy that there 'were giants in those days.' By which I mean, there was a golden age in the past. Frequently things have changed over the centuries but in this case in a bad way.
Some games specifically go against this trope: the rising Empire, an age of glory opening, pushing back the darkness. All that ripe for adventure goodness. A common golden age sci-fi trope come to think of it.
There's been other suggestions for why people keep their game worlds more or less static. From different physics making industrial development difficult to magic making it unnecessary. Of the two I prefer the latter. **
Why develop a steam powered engine when magic does the job already and for less cost? There has to be a tangible reward for the effort of investing in a new technology. The society that invested it's resources (work hours, natural resource, financial, etc) into developing the coal powered steam engine would lose out to the society that had golem power. The only reason to switch over to steam engines would be if golem power proved less viable than coal powered steam. Maybe the golems go on strike or revolt and overthrow their squishy masters or, much more mundane, coal power works out more cost effective.
Ooh, there's a mini-campaign: The golems have been superceded by steam engines and are being 'decommisioned.' And since no-one likes to be euphamised the golems revolt. The PCs can either take the role of golem freedom fighters or Blade Runner-style golem killers. Would that fit into Eberron do you think? Does Eberron have unicorns?
My Dwarven campaign has a great deal of background material. I'll admit I've not given a lot of specific thought to Dwarven technological advances (or lack of same.) But thoughts I have had include: Dwarven reverence for the ancestors and the crafts (and culture in general) as passed down from ancient days encourage a certain cultural conservatism. The long life of the Dwarves also encourages cultural conservatism. There have been changes, but these are rather specific like bigger and better water wheels driving bigger and better machines: quantitative rather than qualitative. Not all changes I've considered have been tech. For instance I've had opera go out of style then come back in a few centuries later. Styles of sculpture and architecture have changed too. Although WHY I would bother to dream this up for a game in which it will never become important I can't say. Call me an 'inveterate world builder.' Or crazy, either is fair.
And as someone said above: half of human history had passed by the time Cyrus conquered the Medes. (!) Yet anyone from that time wouldn't be too out of place in middle ages Europe. Plows are basically still plows, swords are swords. King lives in big castle, peasant in small hovel. Monotheism.
The world changes very slowly. Technological change does gather momentum of course, but it takes a long time to build that momentum.
And here's a thought: many campaigns revolve around the PCs fighting to retain, or recreate, a romanticised, Fabian Socialist Utopia in the face of hostile change. There ya go, I've said it: PCs are agents of cultural conservatism. They're happy to maintain a status quo in which Kings live in castles and peasants in hovels as long as there's lots of phat lewt. Even the precocious, young farm-boy is happy to be a class traitor as long as he gets to marry the princess at the end of it!
*The implications of real world racial determinism in Howard's stuff does make my modern, 21st C sensibilities crawl with discomfort.
** Although tech based on the 4 Aristotlean Elements would be cool.
