Most players I know are terrible at manually fighting. They are under the impression an insanely high attack/defense boost is crucial if you want to win battles, so they focus on increasing that boost and just keep hitting auto battle until they win.
Well, they're not wrong.
The imbalance in this, as I see it, is that the DEF boost for the Attacking player is almost certainly going to be exponentially higher than the ATT boost of the Defending player, which means that, although your high DEF boost reduces the damage your troops take, the Attacker's high DEF boost, faced with the Defender's (probably) very low ATT boost means you're doing even less damage to your opponent.
In my highest DEF City, my DEF boost is just a bit under 600%. But my OFF boost for defending troops is only about 100%. An Attacker with a very high OFF boost might do only 2 or 3 damage per hit to my troops, but I'm probably doing only 1 to his troops so he'll win that battle.