In an interview with Cinema Blend, the director said that since Eternals are synthetic beings who cannot be killed, their necks were saved during the massive snap by Thanos.
No Eternals Were Snapped Away by Thanos - Here's Why.
But it was also the one where Thanos and Hulk's snaps had to happen because that's what led to the stoppage of the Emergence. The real purpose of the Eternals was to follow Arishem's design and allow the Emergence of Tiamut.
They didn't interfere because Thanos is not a Deviant, and the only time they are allowed to intervene in human affairs is when the Deviants are involved. Also they might not have even known what caused the disappearances. Having now seen Eternals I can report that it seems that they were not affected by the Snap.
The Eternals also survived the Blip due to them being artificial and not biological beings.
Tiamut is frozen and killed by being turned to stone - stopping the Emergence.
The Eternals were instructed not to interfere with any human conflict unless Deviants are involved.
In the Eternals movie, Sersi said that the Eternals couldn't fight Thanos because they could only fight in conflicts that involved Deviants.
The Celestials ordered them to ONLY help when Deviants are involved. None of the Avengers' enemies are confirmed to be related to Deviants, hence the Eternals could not help them. Why were there no Avengers during the Eternals timeline?
Technically, Gilgamesh and Sersi were the first Eternals from Earth to become members of the Avengers. But the first Eternal Avenger was none other than Eros, AKA Starfox. Starfox came to Earth in AVENGERS (1963) #232, and he immediately asked to join the roster.
The Celestials are immensely powerful beings featured in the Marvel Comics universe, considered by some to be villains (confirmed as such in a few official works, where they have fought heroes and villains alike, they are greatly feared even by a being of Galactus' might - although in others Galactus has engaged and ...
Doctor Strange watches against threats from other dimensions, but the Eternals are part of our dimension. If it is revealed later that Doctor Strange knows of the Eternals, then the reason he didn't ask them for help in Avengers: Infinity War is that Marvel Studios wasn't ready to introduce the Eternals yet.
Now, it's not explicitly said in Eternals nor in any other MCU movie if Thanos knew about The Emergence in Infinity War or not, but he most likely did.
Thanos killed half everything! Every man, woman, child, animals, plant life, alien beings (including The Celestials). He meant what he said when he would kill half of life within the known universe. No one was excluded except those who survived the outcome.
He felt sad, after all he had to kill his own daughter. But not regret. The “Snap” has always been his goal. And now, he finally achieved it.
How strong they will be in the MCU is anyone's guess, but in the comics, they are weaker than Thanos for sure. The Eternals were effectively wiped out in the comics by their mutant brethren, the Deviants, who were stronger and more powerful than the Eternals.
Physical Strength
The abilities of the Eternals - their strength, speed, power, intelligence, experience - outmatch the Avengers' at almost every turn.
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.
The Celestials didn't interfere because it was not their concern. Their only concern is the Emergence and the birth of new celestials. They must have thought that Thanos' snap might just delay the Emergence because it only takes about 60–70 years to bring the population back to the previous number.
That's why Thanos looks like a giant purple guy; Thanos' Deviant gene deformed his appearance. On the other hand, Eros looks like a regular human, just like other Eternals, because they belong to the same evolutionary path.
Created by Jim Starlin and first appearing in 1973's Invincible Iron Man #55, Eros is Thanos' younger brother.
In the comics, Eternals aren't allowed to conceive a child. But this was proven otherwise when Thanos was born from two Eternals. As a result, he becomes host to the Deviant Gene, which alters his appearance but still technically makes him an Eternal.
Thanos, however, is also afflicted by Deviant syndrome, a genetic disposition that gave him a monstrous appearance but also the power to synthesize cosmic energy, which made him stronger than most Eternals.
They have been specifically instructed not to interfere in human conflicts unless deviants are involved. Also, they were instructed to do so by the Celestials themselves. Since Celestials are similar to Gods, deviants are similar to demons.
Thanos was born as an Eternal but carried a Deviant mutation, meaning he is both Eternal and Deviant.