Honestly, I was thinking the same thing when I first saw TCT's comment, but then again, we know what the 'real' Earth looks like and that doesn't stop us from putting it all in a blender and redoing the map with the Romans, Germans, Japanese, Incans, and Polynesians in a historical game.
For me, at least, a lot depends on the settings/conditions for that type of scenario: How does tech and/or cultural progression work? What about religion? Are key resources (like tin for making bronze) still located in the same place as in the real world, or are their locations randomized on the game map?
Regardless, however, a key difference in most historical 4x games is that due to randomization -- of civilization starting positions, resource locations, etc. -- there is still a reason to include the exploration phase. Without it, you "only" have a grand-strategy game, where you start out in the "expand/exploit" phase. (I find those two typically coexist so thoroughly with each other, that oftentimes they might as well be considered just one "X".)
I'd be curious to know what the tech tree would look like in a Middle Earth game, tho...
That's another question: *Would* there be a tech tree in a Tolkien 4x/GS game?
It's something I've given thought to before, and I have my doubts. Even if there were a tech tree, it would almost have to be a fairly shallow one.
Middle-Earth -- indeed, pretty much all of Arda -- was virtually locked into
Medieval Stasis for thousands of years, since before the beginning of the First Age. There was the *very* occasional mention of more advanced technology (the gunpowder used by Saruman and Gandalf probably being the most prominent example), but that was definitely the exception to the rule.
If a Middle-Earth 4x/GS game were to include a tech tree/research system, it would probably be best to model it on those from the older Total War games, where constructing X, Y, or Z building grants you A, B, or C ability (or unlocks a new, more powerful unit you can recruit into your army). Still not terribly "realistic", but it would at least fit better thematically than the notion of being able to research the telegraph or railroads.