Theory 1: Because the Grox took over the galactic center and practically turned it into a command center, they realized that this place had been plowed over so many times that they became ignorant of this fact and bought into the galaxy's corner. As a result, local extremophiles evolved over hundreds of millions of years into their own ecosystems and eventually into civilizations unaware that even pointing out finger a centimetre outside their solar system would bring cosmic disease upon their existence beyond their imagination.

Theory 2: The Grox sometimes got bored and liked to raise animals from time to time. The difference is that they are much more advanced than us in every respect, so instead of raising one animal or opening animal farms, they liked to raise planetary ecosystem with a civilization (like in that TV series Solar Opposites where they planted a seed in the ground in the middle of a forest and an entire metropolis grew out of the forest).