In Eternals (2021), the Eternals arrival on Earth is in 5000 BC in Mesopotamia, when humans are already well established.
It is only then that the Deviants seem to be a danger (triggering the Eternal's arrival) so where were the Deviants before?
Generally speaking - the Eternals were created as a countermeasure to the Deviants, who somehow must "get" to the planet they put in danger (arrive, exist since the beginning of the planet, ...). Since they are a missed experiment, they are not "exported" to new planets on purpose.
Are the Deviants on each planet that has life? (if all planets were created at the same time that could be plausible) but then why have they waited so long on Earth? And why would they appear on new planets (created by new Celestials)?
Note: sorry for the repetitions of "Eternals" and "Deviants", but I wanted to avoid using "them/they" which I find ambiguous in such a question