Because that's not at all how it works.
This happened to me a couple of years back
I don't believe you.
and I needed to go thought UA.
You did/would not need to do any such thing.
The reason being, UA buys the AE points from Aeroplan and they are the one that must authorize the point transaction.
Regardless of how the back-end transaction works, it needs to be initiated by Aeroplan.