Because *repeats the entire post @sheggaro quoted*
Who said this isn't how they 'thrive'?Fel kills life and defiles the land, but that doesn't mean a demonic civilization can't thrive.
Probably came from other worlds?Where are the machines and cities that supply the Legion with their armaments and ships and warmachines?
Hedonism!? Where did you get this idea that Warcraft demons engage in hedonism?Where do the millions of demons live and engage in hedonism?
I think it's obvious that Legion society doesn't exactly works like what we know a society should.There should be glorious, fel infused cities and buildings. Monuments, industries, you name it!
Unless that counts as paradise to demons?It must really suck to be a demon. You conquer the universe and then you live in a blackened desert. Yay.
Another, as well: do you know how invasions work? You don't begin an invasion by directly attacking the heart of an empire. You start by conquering its borders and moving inwards. In other words: in Argus, we're likely attacking the outskirts of their capital world, not the true heart of the Legion empire.
And another possible theory: after twenty thousand years of constant fel corruption, perhaps all the 'glorious and prosperous' buildings finally corroded out?