It is technically too complex to cut a thread right down to the bottom. If you work with conventional tools, the “blind hole” always remains after creating a thread in a blind hole. The blind hole is created by the tip of the first, “normal” drill , which leaves a conical base. This area cannot be used because taps have a so-called chamfer. The teeth of the tap are flattened in the area of the tip and do not cut a complete thread. The thread areas near the bottom are therefore not usable.
The situation is similar with rubbed holes. The chamfer of the reamer prevents rubbing down to the bottom of the hole.
A core hole in preparation for a blind hole thread must therefore be larger by the depth of the blind hole than the required thread depth. The amount for this depends on the tap used and its chamfer.