Theory and Analysis:S² Engine's Absence in Evangelions

(Rough ideas...got any more?)

S2 engines having to be manufactured and installed by Nerv itself.

Possibilities-

1. Adam and the Angels were already equipped with Super Solenoid engines by the First Ancestral Race themselves. If that were the case, Nerv would have to create its own Super Solenoid engines and fit them to Evangelions.

2. Angels went through a prolonged gestation period in their eggs, during which time they were capable of generating the engines by themselves, while Evas were built in a shorter timeframe and by a process that did not allow for the engines to be created.

3. Evas do have a limited capacity to actually generate energy(As opposed to converting it from one form to the other), as witnessed by Eva-01 and Eva-02 going berserk, but this power capacity can be used only under extreme duress and is inadequate for creating new life or a Third Impact.