Because you do not want to edit opacity. In the context, opacity should be 1. But unreal ties opacity and accumulated distortion, which renders you unable to have any meaningful shading on objects with transmission.
In any case, what OP is asking is largely impractical, as to get it working, you would need to accumulate tint color from all refractive objects, the same way, distortion is accumulated, which is phat.
Gotta settle on using scene color to emissive.