I read the comics so I’m not afraid of spoilers but it’s been a while so remind me if I forgot anything or if they explained it already.
Most empires expand to explore new world, gain strategic resources, visit new things, or even gain new cultures or customs. The viltrumites has been stated as been like the most perfect or powerful species in the world and they already don’t need to really eat are extremely powerful and advanced technology and can freely visit any planet with ease. They don’t care about other cultures, technology, food or knowledge or really need other resources, so what’s the point of expanding? Why have an empire? Why not just explore the galaxy and be not be just absolute bullies? What’s the point?
It’s the whole might makes right/the strong rule the weak Social Darwinist BS. A very primitive way of thinking, ironically. The planets they conquer aren’t “made better” per se, just better for the Viltrumites. They have no real goal apart from ruling over everything and everyone, and if they ever did, they forgot it long ago. This complacency and lack of foresight will cost them big time, just you wait.