Why are child pilots used in the first place? About midway through Evangelion: 2.0 You Can (Not) Advance, Kaji notes that the dummy plug system being introduced is considered more humane than the use of children, after Misato tells him that she distrusts it. But this raises the question: why are children even used as EVA pilots in the first place?

Obviously, from a storytelling perspective, the use of children probably better allows for some of the ideas in the series, such as Asuka and Shinji's parental issues, to be developed. But I do not remember seeing any official, in-universe explanation for this.
Heavy spoilers for the original series:

But if after a decade you haven't still watched the original series, blame on you.

TL;DR:


   Because their moms' souls are what makes the EVA alive. And the EVA activates when the child (pilot) is placed inside the womb (entry plug).


Since the background story is the same for the rebuild movies, we can assume this is unchanged for rebuild.