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?
Yes, but the moment one of them had a clear advantage over the other (proof of being the original/more original) it all went to shit. The Maulers only work when they both don’t know who is the ‘original’.
Now imagine that on a larger scale. The ones at the bottom would be constantly trying to usurp the ones at the top.