Eternals: 24 WTF Moments
23. The Eternals' Flimsy Policy Of Non-Interference
Early in the film, Dane asks Sersi why the Eternals haven't interfered in previous human conflicts, and Sersi informs him that the prime Celestial Arishem (David Kaye) forbade them from getting enmeshed in human affairs unless a Deviant was involved.
Be that as it may, it's a pretty unconvincing explanation all in all - a weak piece of narrative hand-waving to half-heartedly explain why the Eternals didn't show up during the events of prior MCU films, namely Infinity War.
We're probably only going to get more contrived reasoning like this in the future as the MCU becomes more expansive, when honestly the explanations are so piecemeal that the writers might be better off just ignoring it completely at this point.
It's always going to be a problem, yet most fans really don't care that much if the movies are good enough.