Swap ECU question.

[John Campbell]

Why, when doing a swap, can't the factory ECU and harness be used? I have
searched for the answer to this question and have never come across a
thorough answer. I have heard of people doing it, especially with the VVL
swap but that is the exception rather than the rule it seems.
If you have the factory ECU that was programmed by the factory for that
engine and you have the harness that is designed to work with that engine
and ECU and you are using all the sending units/sensors that the ECU is
programmed for it seems to naive me that the whole thing could just be
dropped in (mild oversimplification).
What is it that I'm missing?
This is, of course, if you are only planning on factory hp. But for a VVL or
even a DET swap it seems that it would work, it worked with different sheet
metal wrapped around it.
Yesterday's post about the VVL swap using an SE-R ECU had me baffled.
I'm not being sarcastic, just curious. Can someone school me?

John Campbell
'99 SE with a factory ECU, and HS stuff.