First off, you're wrong about the last part. I've been a quiet reader for a while around here, but I had to finally jump in.
Combat in the real world is extremely terrain-dependent and while not being as much of an issue in close range vs close range combat in the ancient world, getting caught in open terrain by a more mobile attacker or by a ranged attacker IS a death sentence in real world combat, period. Especially so in modern combat.
As far as the games you mention are concerned, I think you're picking the wrong subset of games to use as examples. Risk and Axis & Allies are extremely high-level strategy oriented games. Their rules are based upon the assumption that commanders in the field are making equally sound tactical decisions. They take the tactical out of the equation in favor of focusing solely on strategy. Now, I've never played the last 4 on your list but I assume the rest are in the same vein.
Look at games with more of a lowl-level strategic or tactical element. Play games where terrain actually matters. Where you have to actually worry about supply lines and the like. Or, study history. You brought up WWII europe in a previous post. Try playing a game like WWII: Barbarossa to Berlin. That game essentially has 2 phases and actually follows history very well. In the first phase, the Soviets are the 'defenders' and are at an EXTREME disadvantage. They start with fewer armies that are at a significantly reduced strength compared to the German Panzer Armies. The reason ghe Sovs eventually held was because they didn't sit there and complain that they got butchered on open land (which they did). They understood the tactical advantages to trading territory for time, to shortened supply lines, and extremely importantly to this discussion, using terrain to their advantage. They didn't try and make their stand on the run in open terrain. They made their stands in forests, at river crossings, in cities.
The fact of the matter is in quality wargames there is no individual battle 'defender advantage'. There's an overall strategic advantage as the defender's supply lines get shorter, yes, but that's just as easy to see in Civ 5. In quality wargames, as in the real world, the advantage, initiative, balance of forces, etc is constantly in flux and good commanders/good wargame players constantly use things like terrain, flanking attacks and artillery to their advantage and their opponents' disadvantage.
The issue here is not a defensive modifier for open terrain, except maybe with short range units prior to gunpowder. I can tell you with 100% certainty that open terrain in the more modern eras is downright brutal. The issue is experienced Civ players know how to use terrain to their advantage and for some reason I can't fathom, the AI simply doesn't.
If its anything like civ IV than they probably weigh route choices with maneuverability in mind first and then defensive value.
ex: A swordsman will try to move 2 spaces in a lane of grassland surrounded by forest(as opposed to 1 in grassland) if the city is in the lane or adjacent to it. At the end of each move they will try to end in the highest defensive value tile available(hill with forest for example), but will not go out of their way to do so unless it doesn't alter their travel time negatively.
They don't "think" about being safe from a "potential" enemy attack as opposed to getting to their target ASAP as a priority.