I've never understood why this is true and considered ethical. If a part of your country wishes to govern themselves, or be governed by someone else, why should that be met with violence?

It is a complex system with many actors on different levels.

You can see it on several levels:

- People in power are interested to hold the power as long as they can, and that's why they suppress separatists movements

- Countries which allows such separations become weaker and don't survive, because separation is manipulatable process, and damage country's economy and power

- concept of ethics is very driven by government: they tell people that it is their land, and nobody can take it from them