The Eternals are extremely powerful and with them, avengers can defeat Thanos very easily in the first place. But why they didn’t fight against Thanos till now. Even they never interfere in any fight of humans till now.
Yes, this question has been answered in the final trailer of Eternals. They don’t interfere till now because their priorities were some deviants. The Deviants are a monstrous race of near-immortal beings genetically engineered by the Celestials and the historical enemies of the Eternals, their racial cousins.