Before building up a new frame, it's crucially important to ream/face the headtube and crown race seat (and the bottom bracket, if using press-fit cups). You should also ideally chase the threads of the BB and the derailleur hanger.
But why don't manufacturers (e.g., Surly) do this before they leave the factory? Surly in particular mentions that they do in fact face their head tubes, but they do so before painting the frame, requiring new frame owners to re-face them.
Why wouldn't they just do this before shipping the frame out? It seems like a no-brainer.