system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Eternals Celestial Design

Why do all the celestials we see in Eternals look kind of similar Giant Stone Humanoids with six eyes while Ego and the Head of Knowhere look completely different? 

Also how did Ego not know about this Society of other Celestials? He thought he is the last one.
gpt
I've seen it postulated that Ego Emerged prematurely from a dead, dying, or destroyed planet. It would explain why he instinctively gathered debris to form what would become his planet/body, because the core of his being is encased in a planet again. Ego has no real solid form like Tiamut and Arishem because his egg was destroyed, he's an incomplete Celestial, just a bundle of power wrapped up in some space junk. 

Let us suppose that this is true. Arishem might have just written him off as a lost cause. He can't exactly fulfill the duties of a celestial when he's essentially confined to his planet. He also may not even be powerful enough to generate or renew suns, as if the above theory is correct, then he never absorbed the requisite amount of energy to properly Emerge. This could also explain why Ego, although clearly powerful, is not exactly stable. That's part of why he needed Mantis (disregarding a theory that Mantis is in fact a child of Ego herself) to help him rest peacefully. Perhaps, had his plan succeeded, Ego would have had enough power he would have become a fully powered Celestial, at which point Arishem would definitely take notice of him and probably seek him out.