If it is the idea of conquest you're referring to, it's because they believed in an idea of perfecting China and remaining shut out from the outside world due to a perceived superiority, diplomacy with surrounding nations was mostly just exacting tribute from them and fighting off potential invaders (Mostly from the steppes) and occasionally, as in the case of the British Empire, telling them to go away.

If you meant the idea of the Emperor going to war as a general, this was more to do with the "Mandate of Heaven", the belief that the Emperor was appointed by heaven to rule the world. The problem that arose from this belief was that negative events under the Emperor's rule were perceived as a loss of their mandate and so their dynasty would typically be overthrown, so an Emperor dying in battle would be too great a risk for their successors and therefore going to battle is a bad idea. It's also not necessary since people don't question an Emperor for not going to the battlefield, if the war is won then their mandate is assured.