Why did Ego not know this about Earth?
Ego is a Celestial and plans to terraform Earth, as shown in Guardians of the Galaxy Vol. 2.
But another Celestial, Arishem, has made Earth a host planet for the birth of another Celestial, as mentioned in Eternals.
Why is Ego trying to terraform Earth when it's a host planet that is about to be destroyed? Considering Ego is a Celestial, wouldn't he know?
Even if he knew about it, it's unclear whether the emergence of a new Celestial would have been a problem for Ego's plans.
As explained in The Eternals, the unborn Celestial draws energy from the life living on Earth.
Ajak: Five years ago, Thanos erased half of the population of the universe. But the people of this planet brought everyone back with the snap of a finger. The sudden return of the population provided the necessary energy for the Emergence to begin.
Ikaris: How long do we have?
Ajak: Seven days.
So first of all, given that Ego presumably wasn't planning for the Blip, he probably would have suspected he had longer before the Emergence. In addition the process of terraforming would have killed all life on Earth (and all the other planets he seeded), cutting off the Celestial's food source. The Celestial clearly wasn't immortal either, so it's entirely possible it would have been killed by the process too.
Pictures about "Why did Ego not know this about Earth?"
Why was Ego different from other Celestials?Ego was a celestial born outside of the influence of another celestial (i.e. he wasn't born of a celestial seed like the ones we now know are) and thus, lacked the typical physical form of one. He didn't have a planet, or sentient energy to fee off of to grow and mature like a celestial born of a seed would.
Why did Ego destroy the universe?A living planet with a humanoid extension of himself, Ego sought to find meaning in his life and, to achieve this end, planned to remake the entire universe via an omnicidal extinction-level event known as the Expansion, using "seeds" planted on various worlds to terraform them into new extensions of himself, thereby ...
Is Ego just a planet?All you really need to know about Ego the Living Planet is right there in his name. He's basically a giant space rock that gained sentience and now travels the universe stirring up mischief.
ETERNALS: Ego Was Created to Defeat Galactus | Marvel Theory Explained
More answers regarding why did Ego not know this about Earth?
One interpretation would be that Ego simply didn't care. His master plan would have decimated life in the galaxy. So this incarnation of Ego seemed to have little or no respect for life other than his own.