Having a standing army was considered the norm for most of China's history. The issue is that usually the standing army stations at either the borders, or at t

Having a standing army was considered the norm for most of China's history. The issue is that usually the standing army stations at either the borders, or at the capital. Rebellions usually start at some place that's far away from where most of the standing army is stationed. Nomadic empires were effective enemies because they had the "optionality" to wait until the agricultural empire was weak (for whatever reason), and then strike. The nomads don't get wiped out at bad times because nomadic tribes don't have cities or capitals that can be raided or sacked, and they usually live in a vast depopulated land. Also, after a long time of peace, nobody knows how to fight battles in agricultural societies (or at least there's no way for military officers to prove themselves), and then one or two bad military appointments could just end a civilization. Nomadic people at least get ample training on how to ride horses and hunt.