Probably because success and failure are both positive feedback loops. You build an army, conquer a city, which gives you more resources so you build a second army, so you conquer two more cities...etc. failure is you lose a city, so you can't support as many armies, so you can't defend as many cities, so this time you lose two.. Positive feedback loops are not good for gameplay. On the other hand, it's hard to avoid them because why would invade another city, if not to gain a further advantage? I'm playing GalCiv3 and I couldn't tell you how the mechanics brought this on, but capturing colonies isn't producing a massive runaway effect like it does in Civ VI. They're a boon to be sure, but even now that I've taken a third of the map, while I'm the leading faction, I haven't reach god tier either. The corollary is that if I were to suffer a reversal, it wouldn't automatically spell my doom I'd have a chance of stopping the rot and reversing the tide. Of course, the other issue is time. You spend hours building up your civ, if you lose it tona substantial degree, frustration is inevitable.