Here is the answer to why too games are extremely short: the complete destruction of defensive play through towers and the ‘ballistics’ upgrade, which allows defenses to hit moving targets.
However, the population limit is another problem. In the mid-game (before creating caravans) is when the largest armies are seen, and they rarely exceed 40 units (In other RTS games like AoE, they don’t drop below 60-80 at a minimum).
I discussed the problem in detail HERE if you’d like to read it.