There might need to be some testing done before the work was completed.
Likes of a Hi-Pot test or what-ever.
The reason the terminations are made like that before all the mechanical stuff is finished is pure economics, maybe the test technician could only be at that site at that time.
Sure, we would all like to have things finished, I've tested work in sub-stations here where the engineering part has not been completed, but is safe.
Commissioning HV gear is in itself a long slow process.