I totally see the logic of your question, especially when popular media tends to inundate us with images of unstoppable nomad conquerors like Chingghis Khaan, Attila the Hun, Tamerlane, Nader Shah, Modun Chanyu, and Tanshihuai, but the truth is that pastoralists (really, hybrid agro-pastoralists, few societies in history have been purely pastoralists) really weren’t a force of nature. They won many spectacular victories, of course. But their long-term strength wasn’t in being able to build empires from the Caspian to the Sakhalin, which settled states could also do and often better, but rather in being able to recover very quickly from defeats by settled states, which allows us to sweep those less impactful episodes under the rug until the last few hundred years.

And… they had a rather high figure for defeats, in the long run. The most recognizable to Westerners is likely the Battle of the Catalaunian Plains between Rome and the Hunnic Diarchy, which was really more of a draw, but by no means a victory for Attila. The most decisive is likely Tang Taizong’s conquest of the Goktürk Khaganate, arguably the largest polity on earth at the time.

And it wasn’t alone; Taizong also bested and conquered the Tuyuhun and Sir-Yantuo confederations. China had a remarkable history of doing so, from Cao Cao’s eradication of the Wuhuan to the Ming emperor Yongle’s highly successful five Mobei Campaigns. More under the surface, border policy from agrarian and semi-agrarian states in the region consistently prevented the rise of legitimately continent-threatening nomadic confederacies after the fall of the Uyghurs (with the noted exception of the Mongols) by backing different actors at different times.

And this wasn’t just China. In Iran, the Romans frequently beat back the semi-nomadic Parthian Empire, which was eventually toppled by an agrarian Persian revolt. In Europe, the Duchy of Muscovy—eventually known as Russia—spent much of its career smashing pastoralist polities like the Khanates of Sibir, Kazan, Astrakhan, the Kazakh, Khiva, and others. In the Sahel, despite the context of the nomadic Fula jihads, the Toucouleur Empire still wiped out the Fula Massina state and the Waddai Empire ended the Kanem-Bornu confederation.

To conclude: the answer to the question is that there were very clear precedents of nomads not being unstoppable forces of nature. When nomads won they often won by a lot, but that doesn’t mean they won all the time.

There’s also that, y’know, people generally don’t like being invaded.