Every great empire has to start as a small one, so <insert small polity> becoming a great empire isn’t ASB.
And many great empires formed on the periphery between the sedentary and non sedentary populations, so groups like the Arabs are also unsurprising, in general. Meanwhile, the Mongols were just the most successful in a long string of nomad empires. The US growing to global dominance? People were predicting that before 1776.
Sure, there’s plenty of black swan events, but they’re only outlandish before they happen, and completely reasonable after.
I can’t come up with a satisfactory answer, myself.