Why is that? Pan civ trade routes balance each other out, which leaves no advantage to one civ or the other. The revenue recieved from caravans isn't too substantial until automobile, at which point it's easy to patrol for and destroy any oncoming freight units (trusting you aren't directly connected to your opponent by rail). It does have massive benefits when an economic alliance is set up to exclude other civs (in a larger game), but then it's simply the job of the other civs to do the same; and to achieve a balance of power.
Of course you do recieve self beneficial results when you have much more infrastructure than your opponents (ie. trade multipliers), but a good opponent would not let that happen
