They don't go on, like, random road trips. They migrate according to climate conditions so they can feed their flocks. 2. Nomads don't generally produce manufactured goods, which means they need to trade, so they almost always live near settled people. And 3. Because they generally live close to nature and in harsh conditions, pastoralists tend to be tougher than diamond-plated differential calculus. Like think of the Huns, or the Xiongnu, or the Mongols.