It's possible that it's simply too easy to defend under the current balance.
I find that defending gets considerably easier the later the game goes on. In large part this is due to the very high military cap that appears later on in the game - as Tradition I often have little to no trouble at all defending even a fairly large area with a meagre number of units against the onslaught of a warmonger with upteen cities - ultimately, the warmonger may have tons more military, but is unable to use them against me in any meaningful way because he can only fit 1 unit per tile. Earlier in the game when it's more of a hassle to train units in general, it's easy to be surrounded when fighting a meagre opponent's army when unprepared.
I'd figure that if the unit cap were reduced proportionally all around (e.g. warmongers have fewer units but can actually use them all; a tall civ's cap is so low that it is difficult to get enough units to actually patrol the border), I think we'd see much more interesting warfare. This is besides the point that fighting walls of units is boring and makes turns take forever...so that just adds on to the "late game" discussions that some people have had and why we get games of the first 150 turns so much more often - because the first 150 turns of warfare is just more fun but also more balanced.