Maybe because a single coil is wound directly around the polepieces with a vulcanised fibre top and bottom added, the screw holes are fitted to the base of this bobbin. Whereas a humbucker is wound on a plastic bobbin and this is then attached to a metal plate. The screw holes are on lugs on the metal plate at the midpoint where the two bobbins of the HB meet.
In simple terms the holes are in the wrong place. I'm sorry if this is too simple or seems patronising.
I suppose what could be done is one coil of a humbucker could be wound and the other left empty, but a plastic humbucker bobbin would probably only accomodate screws or metal slugs with a bar magnet underneath rather than actual alnico magnet polepieces, so the sound would be more like a P90 than a true single coil.