A country is different from a sect. If you destroy a sect, it is truly gone. Even a large sect has at most a few tens of thousands of members, or perhaps a little over a hundred thousand at the absolute peak.
A country, however, is not made up of hundreds of thousands of people; it consists of hundreds of millions. A massive nation like the Xuanwu Empire boasts a population counted in the hundreds of millions.
With a population of that magnitude, any retaliation would be endless and impossible to guard against. No matter how powerful you might be, every single person around you cannot possibly be strong.
Chapter 370