I mean the thread which is exposed to load and hence stretching.
With a normal bolt the only bit that is exposed to that load is the bit where it passes through the wheel, which is only about half a dozen mm thick from the bottom of the taper to the back face of the wheel, if you add a 20mm spacer though the bolt is exposed all the way through that as well, so issues like them loosening when they get hot (due to stretch) get far more prominient.
On a bolt to hub spacer instead of one very long section of exposed thread, you have two bolts each with a short section still. (*5 obviously, Im talk in terms of per bolt hole)