Celestials made some decisions a million years ago.
The Eternals were a slavish defence mechanism for the Celestial foetus playing with itself underground and the Deviants were party people. The cavemen seeded with the potential for mutation down the line were just supposed to be footsolders to fight off Galactus who in truth doesn't eat the life force of planets but Celestial foetuses infesting planets.
Eugenics, not evolution.
Mutation was going to happen no matter what because Galactus was always coming..
Evolution was a slow plod to Eternal status which might take another billion years unless we evolve down a dead end; like the Kree or the Skrulls if you beleive in that claremont Annnual in the 80s that supposes there are gods looking over Mutation, Evolution and Ceasation.
We can't become Eternals if Mutation turns mankind into Deviants.
Would you rather be thelife of the party or perfectly dull?