The headstock is line bored to ensure that the spindle is perfectly aligned to the bed. It is the simplest and fastest way get it accurately aligned and does not require the use of indicators etc... If you machine the bore separately then you need to align it to the bed in multiple axes. That is not an easy task and you would probably not get it as well aligned. It is actually less work to build the temporary headstock than to try to align it otherwise.
bob